- refine/define Scope, Object, and Type structures
  (note: scope.go has the addition of types, the rest is only re-organized
  for better readability)
- implemented top-level of type checker:
  resolve global type declarations (deal with double decls, cycles, etc.)
- temporary hooks for checking of const/var declarations, function/method bodies
- test harness for fine-grained testing (exact error locations)
  with initial set of tests

This is a subset of the code for easier review.

R=rsc
CC=golang-dev
https://golang.org/cl/1967049