disambiguate defaults by class

Defining multiple defaults for the same method will clobber eachother, even if they utilize different sublcass methods. This leads to a nasty nonlocal bug where an upstream module may fail to compile if it is imported together with a module that clobbers the default it wants, even though locally that upstream module imports the right defaults.

Add an extra `Name` argument to `defaulting` which takes the name of the subclass providing the default, and disambiguate using it in `instances`