SE-0402: Generalize `conformance` macros as `extension` macros

[hborla (Holly Borla)]

@Dan_Stenmark

would the macro engine also permit expanding code into extensions on the protocol itself where Self is the implementor?

No, this proposal only allows extending the type the macro is attached to. Extending other types is a future direction, but all extended types would need to be listed in the @attached(extension) attribute so they're known prior to macro expansion. This came up in the pitch thread too:

@hborla

@stephencelis

This looks like a good improvement! Would it also pave the way for lifting some of the restrictions on extensions produced by macros?

Possibly. I could imagine a future addition to @attached(extension) that allows you to name the extended type in the macro attribute, e.g. @attached(extension, of: MyType), which defaults to the type the macro is attached to.