Implement the has_one_attached and has_many_attached macros, using the same semantics as ActiveStorage. (Make sure differences in the API are absolutely necessary, and document them thoroughly if so.)

They are, fundamentally, just specializations of has_one and has_many, and it may be possible to implement them as simple wrappers around those macros. In order to be able to reflect on attachments, however, it will also be necessary to add a separate list that tracks attachment associations, which will be appended to by has_one_attached and has_many_attached.