So far, only top-level names where considered when trimming ASTs
using a filter function. For instance, "godoc reflect Implements"
didn't show the "Implements" method of the type Interface because
the local method name was not considered (on the other hand, "top-
level" declared methods associated with types were considered).

With this CL, AST filter functions look also at struct fields
and interface methods.

R=rsc, r
CC=golang-dev
https://golang.org/cl/4518050