freeze the column name to drop string allocations in dirty checks

Dirty checking keeps a hash where the keys are the column name and the
value is a dup of the value from the database[1].  This hash is kept for
every AR object, which means that we dup every column name for every AR
object that does dirty checking.  Freezing the column name prevents the
column name from being duped and reduced overall string allocations.

Here is a benchmark to demonstrate:

```ruby
require 'active_record'

class Topic < ActiveRecord::Base
end

20.times do |i|
  Process.waitpid fork {
    ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'

    ActiveRecord::Base.connection.instance_eval do
      create_table(:topics) do |t|
        t.string   :title, limit: 250
        t.string   :author_name
        t.string   :author_email_address
        t.string   :parent_title
        t.string   :type
        t.string   :group
        i.times do |j|
          t.string :"aaa#{j}"
        end
        t.timestamps null: true
      end
    end

    ObjectSpace::AllocationTracer.setup(%i{type})

    Topic.create title: "aaron" # heat cache

    result = ObjectSpace::AllocationTracer.trace do
      10.times do |i|
        Topic.create title: "aaron #{i}"
      end
    end

    puts "#{Topic.columns.length},#{(result.find { |k,v| k.first == :T_STRING }.last.first / 10)}"
  }
end
```

1. https://github.com/rails/rails/blob/3ad381c3f8598d9920998c8949a96b5f62b280dd/activerecord/lib/active_record/attribute_set/builder.rb#L102

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