We're trying to make instantiating models cheaper (for example doing
`Post.new`), and discovered in the course of profiling that
`respond_to?` is responsible for some amount of initialization
allocations.

This patch changes `Model.respond_to?` to not allocate anymore which
should help initialization performance (as well as other queries).

Here is the benchmark we used:

```ruby
require "active_record"

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

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
    t.string :title, default: "hello"
    t.text :body, default: "i am a blog post"
    t.string :author, default: "aaron"
    t.boolean :published, default: true
    t.integer :likes, default: 1000000
  end
end

class Post < ActiveRecord::Base
end

5.times { Post.new }

def m
  x = GC.stat(:total_allocated_objects)
  yield
  GC.stat(:total_allocated_objects) - x
end

allocs = m { 5000.times { Post.new } }
p ALLOCATIONS_PER_MODEL: (allocs / 5000)

allocs = m { 5000.times { Post.respond_to?(:default_scope) } }
p ALLOCATIONS_PER_RESPOND_TO: (allocs / 5000)
```

Before this patch:

```
$ bundle exec ruby -Iactiverecord/lib:~/git/vernier/lib test.rb
-- create_table(:posts, {force: true})
   -> 0.0045s
   {ALLOCATIONS_PER_MODEL: 9}
   {ALLOCATIONS_PER_RESPOND_TO: 2}
```

After this patch:

```
$ bundle exec ruby -Iactiverecord/lib:~/git/vernier/lib test.rb
-- create_table(:posts, {force: true})
   -> 0.0045s
   {ALLOCATIONS_PER_MODEL: 7}
   {ALLOCATIONS_PER_RESPOND_TO: 0}
```

Co-Authored-By: Eileen M. Uchitelle <eileencodes@gmail.com>