Reduce allocations when running AR callbacks.

Inspired by @tenderlove's work in
c363fff2, this reduces the number of
strings allocated when running callbacks for ActiveRecord instances. I
measured that using this script:

```
require 'objspace'
require 'active_record'
require 'allocation_tracer'

ActiveRecord::Base.establish_connection adapter: "sqlite3",
                                        database: ":memory:"

ActiveRecord::Base.connection.instance_eval do
  create_table(:articles) { |t| t.string :name }
end

class Article < ActiveRecord::Base; end
a = Article.create name: "foo"
a = Article.find a.id

N = 10
result = ObjectSpace::AllocationTracer.trace do
  N.times { Article.find a.id }
end

result.sort.each do |k,v|
  p k => v
end
puts "total: #{result.values.map(&:first).inject(:+)}"
```

When I run this against master and this branch I get this output:

```
pete@balloon:~/projects/rails/activerecord$ git checkout master
M Gemfile
Switched to branch 'master'
pete@balloon:~/projects/rails/activerecord$ bundle exec ruby benchmark_allocation_with_callback_send.rb > allocations_before
pete@balloon:~/projects/rails/activerecord$ git checkout remove-dynamic-send-on-built-in-callbacks
M Gemfile
Switched to branch 'remove-dynamic-send-on-built-in-callbacks'
pete@balloon:~/projects/rails/activerecord$ bundle exec ruby benchmark_allocation_with_callback_send.rb > allocations_after
pete@balloon:~/projects/rails/activerecord$ diff allocations_before allocations_after
39d38
<
{["/home/pete/projects/rails/activesupport/lib/active_support/callbacks.rb",
81]=>[40, 0, 0, 0, 0, 0]}
42c41
< total: 630
---
> total: 590

```

In addition to this, there are two micro-optimizations present:

* Using `block.call if block` vs `yield if block_given?` when the block was being captured already.

```
pete@balloon:~/projects$ cat benchmark_block_call_vs_yield.rb
require 'benchmark/ips'

def block_capture_with_yield &block
  yield if block_given?
end

def block_capture_with_call &block
  block.call if block
end

def no_block_capture
  yield if block_given?
end

Benchmark.ips do |b|
  b.report("block_capture_with_yield") { block_capture_with_yield }
  b.report("block_capture_with_call") { block_capture_with_call }
  b.report("no_block_capture") { no_block_capture }
end
pete@balloon:~/projects$ ruby benchmark_block_call_vs_yield.rb
Calculating -------------------------------------
block_capture_with_yield
                        124979 i/100ms
block_capture_with_call
                        138340 i/100ms
    no_block_capture    136827 i/100ms
-------------------------------------------------
block_capture_with_yield
                      5703108.9 (±2.4%) i/s -   28495212 in   4.999368s
block_capture_with_call
                      6840730.5 (±3.6%) i/s -   34169980 in   5.002649s
    no_block_capture  5821141.4 (±2.8%) i/s -   29144151 in   5.010580s
```

* Defining and calling methods instead of using send.

```
pete@balloon:~/projects$ cat benchmark_method_call_vs_send.rb
require 'benchmark/ips'

class Foo
  def tacos
    nil
  end
end

my_foo = Foo.new

Benchmark.ips do |b|
  b.report('send') { my_foo.send('tacos') }
  b.report('call') { my_foo.tacos }
end
pete@balloon:~/projects$ ruby benchmark_method_call_vs_send.rb
Calculating -------------------------------------
                send     97736 i/100ms
                call    151142 i/100ms
-------------------------------------------------
                send  2683730.3 (±2.8%) i/s -   13487568 in   5.029763s
                call  8005963.9 (±2.7%) i/s -   40052630 in   5.006604s
```

The result of this is making typical ActiveRecord operations slightly faster:

https://gist.github.com/phiggins/e46e51dcc7edb45b5f98

activemodel/lib/active_model/validations.rb

@@ -390,7 +390,7 @@ def invalid?(context = nil)
   protected
 
     def run_validations! #:nodoc:
-      run_callbacks :validate
+      run_validate_callbacks
       errors.empty?
     end
   end

activemodel/lib/active_model/validations/callbacks.rb

@@ -108,7 +108,7 @@ def after_validation(*args, &block)
 
       # Overwrite run validations to include callbacks.
       def run_validations! #:nodoc:
-        run_callbacks(:validation) { super }
+        run_validation_callbacks { super }
       end
     end
   end

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -159,7 +159,7 @@ def delete_records(records, method)
               count = scope.destroy_all.length
             else
               scope.to_a.each do |record|
-                record.run_callbacks :destroy
+                record.run_destroy_callbacks
               end
 
               arel = scope.arel

activerecord/lib/active_record/callbacks.rb

@@ -289,25 +289,25 @@ module ClassMethods
     end
 
     def destroy #:nodoc:
-      run_callbacks(:destroy) { super }
+      run_destroy_callbacks { super }
     end
 
     def touch(*) #:nodoc:
-      run_callbacks(:touch) { super }
+      run_touch_callbacks { super }
     end
 
   private
 
     def create_or_update #:nodoc:
-      run_callbacks(:save) { super }
+      run_save_callbacks { super }
     end
 
     def _create_record #:nodoc:
-      run_callbacks(:create) { super }
+      run_create_callbacks { super }
     end
 
     def _update_record(*) #:nodoc:
-      run_callbacks(:update) { super }
+      run_update_callbacks { super }
     end
   end
 end

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -360,7 +360,7 @@ def checkin(conn)
         synchronize do
           owner = conn.owner
 
-          conn.run_callbacks :checkin do
+          conn.run_checkin_callbacks do
             conn.expire
           end
 
@@ -449,7 +449,7 @@ def checkout_new_connection
       end
 
       def checkout_and_verify(c)
-        c.run_callbacks :checkout do
+        c.run_checkout_callbacks do
           c.verify!
         end
         c

activerecord/lib/active_record/core.rb

@@ -272,7 +272,7 @@ def initialize(attributes = nil, options = {})
       init_attributes(attributes, options) if attributes
 
       yield self if block_given?
-      run_callbacks :initialize unless _initialize_callbacks.empty?
+      run_initialize_callbacks
     end
 
     # Initialize an empty model object from +coder+. +coder+ must contain
@@ -294,8 +294,8 @@ def init_with(coder)
 
       self.class.define_attribute_methods
 
-      run_callbacks :find
-      run_callbacks :initialize
+      run_find_callbacks
+      run_initialize_callbacks
 
       self
     end
@@ -331,7 +331,7 @@ def initialize_dup(other) # :nodoc:
       @attributes = @attributes.dup
       @attributes.reset(self.class.primary_key)
 
-      run_callbacks(:initialize) unless _initialize_callbacks.empty?
+      run_initialize_callbacks
 
       @aggregation_cache = {}
       @association_cache = {}

activerecord/lib/active_record/transactions.rb

@@ -309,7 +309,7 @@ def rollback_active_record_state!
     # Ensure that it is not called if the object was never persisted (failed create),
     # but call it after the commit of a destroyed object.
     def committed!(should_run_callbacks = true) #:nodoc:
-      run_callbacks :commit if should_run_callbacks && destroyed? || persisted?
+      run_commit_callbacks if should_run_callbacks && destroyed? || persisted?
     ensure
       force_clear_transaction_record_state
     end
@@ -317,7 +317,7 @@ def committed!(should_run_callbacks = true) #:nodoc:
     # Call the +after_rollback+ callbacks. The +force_restore_state+ argument indicates if the record
     # state should be rolled back to the beginning or just to the last savepoint.
     def rolledback!(force_restore_state = false, should_run_callbacks = true) #:nodoc:
-      run_callbacks :rollback if should_run_callbacks
+      run_rollback_callbacks if should_run_callbacks
     ensure
       restore_transaction_record_state(force_restore_state)
       clear_transaction_record_state

activesupport/lib/active_support/callbacks.rb

@@ -78,18 +78,21 @@ module Callbacks
     #     save
     #   end
     def run_callbacks(kind, &block)
-      cbs = send("_#{kind}_callbacks")
-      if cbs.empty?
-        yield if block_given?
+      send "run_#{kind}_callbacks", &block
+    end
+
+    private
+
+    def _run_callbacks(callbacks, &block)
+      if callbacks.empty?
+        block.call if block
       else
-        runner = cbs.compile
+        runner = callbacks.compile
         e = Filters::Environment.new(self, false, nil, block)
         runner.call(e).value
       end
     end
 
-    private
-
     # A hook invoked every time a before callback is halted.
     # This can be overridden in AS::Callback implementors in order
     # to provide better debugging/logging.
@@ -722,6 +725,12 @@ def define_callbacks(*names)
         names.each do |name|
           class_attribute "_#{name}_callbacks"
           set_callbacks name, CallbackChain.new(name, options)
+
+          module_eval <<-RUBY, __FILE__, __LINE__ + 1
+            def run_#{name}_callbacks(&block)
+              _run_callbacks(_#{name}_callbacks, &block)
+            end
+          RUBY
         end
       end