compiler: reimplement interface type asserts

[aykevl]

This is a big reimplementation that simplifies the compiler a lot. Instead of storing the method set in metadata and lowering the type asserts as a whole program pass, this change just puts the list of methods in the type code (and a separate global for the interface type).

I'm still working on this. I'm not very happy with the binary size increase (~4%) but I have a few ideas how to reduce it. But I made a PR for initial review for those interested (@dgryski?)

This is also a step in the direction of #2870.

[dgryski]

Are you still thinking of working on this?

[reteps]

cc @aykevl -- my assumption is no?