Chan/port support in frontend; typechecking work.

- Support chans as well as ports by use of 'chan' keyword.

- Actually hooked up frontend to backend! Starting to work through the
  resulting type-mismatches that this exposes in the generated IR (e.g.,
  spawn's type == spawn-ID).

- Some work in typechecking (frontend/AST and backend/IR) to help debug
  this.