go/importer: added go/importer package, adjusted go/types

go/importer: added go/importer package, adjusted go/types

- The go/importer package provides access to compiler-specific importers.
- Adjusted go/internal/gcimporter and go/types as needed.
- types.Check was removed - not much simpler than calling types.Config.Check.
- Package "unsafe" is now handled by the type checker; importers are not
  called for it anymore.
- In std lib tests, re-use importer for faster testing
  (no need to re-import previously imported packages).
- Minor cleanups.

The code still needs cleanups before submitting.

Change-Id: Idd456da2e9641688fe056504367348926feb0755
Reviewed-on: https://go-review.googlesource.com/8767
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>