Method macros

This patch adds support for macros in method position. It follows roughly the template for Item macros, where an outer `Method` wrapper contains a `Method_` enum which can either be a macro invocation or a standard macro definition. 

One note; adding support for macros that expand into multiple methods is not included here, but should be a simple parser change, since this patch updates the type of fold_macro to return a smallvector of methods.

For reviewers, please pay special attention to the parser changes; these are the ones I'm most concerned about.

Because of the small change to the interface of fold_method, this is a ...

[breaking change]