Follow-up to #39147 and #39168.

By adding a new purpose-specific format, we avoid potential pitfalls
from concatenating format strings.  We also save a String allocation per
Time attribute per inspect.

The new format also includes a time zone offset for more introspective
inspection.

DATE_FORMATS can be either a format string or a lambda we don't want to attempt to concat them with a string if we are in the lambda case

before
```
p Knot.create
=> #<Knot id: 1, created_at: "2016-05-05 01:29:47">
```

after
```
p Knot.create
=> #<Knot id: 1, created_at: "2016-05-05 01:29:47.116928000">
```

Since ruby/ruby@5208c431be, `Time#inspect` is separated from
`Time#to_s`, and the former may contain the fraction part now.

The `read_attribute` method always returns the primary key when asked to
read the `id` attribute, even if the primary key isn't named `id`, and
even if another attribute named `id` exists.

For the `inspect`, `attribute_for_inspect` and `pretty_print` methods,
this behaviour is undesirable, as they're used to examine the internal
state of the record. By using `_read_attribute` instead, we'll get the
real value of the `id` attribute.

In Ruby 2.3 or later, `String#+@` is available and `+@` is faster than `dup`.

```ruby
# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips"
end

Benchmark.ips do |x|
  x.report('+@') { +"" }
  x.report('dup') { "".dup }
  x.compare!
end
```

```
$ ruby -v benchmark.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
Warming up --------------------------------------
                  +@   282.289k i/100ms
                 dup   187.638k i/100ms
Calculating -------------------------------------
                  +@      6.775M (± 3.6%) i/s -     33.875M in   5.006253s
                 dup      3.320M (± 2.2%) i/s -     16.700M in   5.032125s

Comparison:
                  +@:  6775299.3 i/s
                 dup:  3320400.7 i/s - 2.04x  slower

```

Some places we can't remove because Ruby still don't have a method
equivalent to strip_heredoc to be called in an already existent string.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

The methods got moved to core.rb in commit
b2c9ce34, but the tests never did.