Adding tostring and _torepr methods for sets.  (Ironically, no longer needs the fix in the previous commit, but it's better that that's been fixed, anyway...)

(cherry picked from commit 2ea4ab85a7f8a52cee1e81105e169a2d0116f121)
Adds plumbing of s-module form, instead of desugaring modules into Pyret objects immediately.  (We still compile the new module form into a Pyret object, because that's what the runtime currently expects, but that can now change more easily.)
Grammatically restricting predicate annotations to be identifiers, rather than full expressions