Prefer faster `str.start_with?` over `str.first ==`

```ruby
str = "abc"

Benchmark.ips do |x|
  x.report("start_with?") { str.start_with?("a") }
  x.report("first ==")    { str.first == "a" }
end
```

```
Warming up --------------------------------------
         start_with?   282.381k i/100ms
            first ==   207.305k i/100ms
Calculating -------------------------------------
         start_with?     10.239M (± 2.2%) i/s -     51.393M in   5.022151s
            first ==      4.593M (± 4.5%) i/s -     23.011M in   5.021434s
```

actionpack/lib/abstract_controller/translation.rb

@@ -11,7 +11,7 @@ module Translation
     # to translate many keys within the same controller / action and gives you a
     # simple framework for scoping them consistently.
     def translate(key, **options)
-      if key.to_s.first == "."
+      if key.to_s.start_with?(".")
         path = controller_path.tr("/", ".")
         defaults = [:"#{path}#{key}"]
         defaults << options[:default] if options[:default]

actionpack/lib/action_dispatch/testing/assertions/routing.rb

@@ -89,7 +89,7 @@ def assert_generates(expected_path, options, defaults = {}, extras = {}, message
             expected_path = uri.path.to_s.empty? ? "/" : uri.path
           end
         else
-          expected_path = "/#{expected_path}" unless expected_path.first == "/"
+          expected_path = "/#{expected_path}" unless expected_path.start_with?("/")
         end
         # Load routes.rb if it hasn't been loaded.
 
@@ -210,7 +210,7 @@ def recognized_request_for(path, extras = {}, msg)
               request.path = uri.path.to_s.empty? ? "/" : uri.path
             end
           else
-            path = "/#{path}" unless path.first == "/"
+            path = "/#{path}" unless path.start_with?("/")
             request.path = path
           end
 

actionview/lib/action_view/helpers/translation_helper.rb

@@ -123,7 +123,7 @@ def localize(object, **options)
       private
         def scope_key_by_partial(key)
           stringified_key = key.to_s
-          if stringified_key.first == "."
+          if stringified_key.start_with?(".")
             if @virtual_path
               @_scope_key_by_partial_cache ||= {}
               @_scope_key_by_partial_cache[@virtual_path] ||= @virtual_path.gsub(%r{/_?}, ".")