Add bigint primary key support for MySQL.

This PR will support bigint primary key for MySQL, such as bigserial for PostgreSQL.

Example:

```ruby
create_table :foos, id: :bigint do |t|
end
```
Yes, please.

On Sat Dec 27 2014 at 8:03:04 AM Ryuta Kamizono <notifications@github.com>
wrote:

> In activerecord/test/cases/primary_keys_test.rb
> <https://github.com/rails/rails/pull/18220#discussion-diff-22292859>:
>
> > +
> > +    teardown do
> > +      @connection.drop_table :widgets
> > +    end
> > +
> > +    def test_bigint_primary_key
> > +      assert_equal "id", Widget.primary_key
> > +
> > +      column = Widget.columns_hash[Widget.primary_key]
> > +      assert_equal :integer, column.type
> > +      assert_equal 8, column.limit
> > +
> > +      widget = Widget.create!
> > +      assert_not_nil widget.id
> > +    end
> > +  end
>
> Yes, only differ whether :bigint or :bigserial, test cases are the same.
> Should I eliminate duplication of this test cases?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/rails/rails/pull/18220/files#r22292859>.
>