- Changed Check signature to take function parameters for
  more flexibility: Now a client can interrupt type checking
  early (via panic in one the upcalls) once the desired
  type information or number of errors is reached. Default
  use is still simple.

- Cleaned up main typechecking loops. Now does not neglect
  _ declarations anymore.

- Various other cleanups.

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/6612049