go/types: Moving from *ast.Objects to types.Objects (step 2).

Completely removed *ast.Objects from being exposed by the
types API. *ast.Objects are still required internally for
resolution, but now the door is open for an internal-only
rewrite of identifier resolution entirely at type-check
time. Once that is done, ASTs can be type-checked whether
they have been created via the go/parser or otherwise,
and type-checking does not require *ast.Object or scope
invariants to be maintained externally.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/7096048