pkg/runtime: implement time.now() in C
Generate more runtime type info

Also, handle recursive types while
generating runtime type info.
pkg/runtime: check function types in interface comparisons
Update to latest go/types

- Use NewParam in lieu of removed NewFieldVar,
- Remove unused vars from runtime code,
  as they now cause an error in go/types.
pkg/runtime: revert time.now change

Needs some more thought for cross-platform,
backwards-compatibility.