go/doc: streamlined go/doc API

- the main changes are removing the Doc suffix
  from the exported types, so instead of
  doc.TypeDoc one will have doc.Type, etc.

- All exported types now have a Name (or Names) field.
  For Values, the Names field lists all declared variables
  or constants.

- Methods have additional information about where they are
  coming from.

- There's a mode field instead of a bool to
  control the package's operation, which makes
  it easier to extend w/o API changes.

Except for the partially implemented new Method type,
this is based on existing code. A clean rewrite is in
progress based on this new API.

R=rsc, kevlar
CC=golang-dev
https://golang.org/cl/5528060