Ponyc assertion on funky lambda capabilities

[cquinn]

Whilst randomly trying to fix capability errors in my code, I ended up with a combination that causes ponyc to assert. This is the latest master ponyc built and run on MacOS.

I've narrowed the case down to the following snippet (which most likely has errors, but shouldn't break the compiler):

class val Thing
  let result: {(Bool val)}

  new val bool(result': {(Bool)} val = {(Bool)=>None}) ? =>
    result = result'

[SeanTAllen]

Error is:

src/libponyc/pass/scope.c:19: set_scope: Assertion `ast_id(name) == TK_ID` failed.

[cquinn]

Oops, I forgot to include the error. That's what I see too.

[SeanTAllen]

This is fixed and now gives a proper error.