Methods to get accessors related to is_many accessor

This extracts the name resolution for property names related to is_many accessors.  With these, you can ask the class object what the iterator accessor is for a given is_many property name.
This isn't something changing in this diff, but the documentation and method names are careful to not call the adder and the remover "accessors".  It's a little inconsistent that these variable names are then `$add_accessor_name` and `$remove_accessor_name`.