- enable AllMethods flag (default: not set)
- fix logic determining which methods to show
- added respective test case in testdata/e.go for AllMethods = false
- added test case set for AllMethods = true

The critical changes/files to look at are:
- testdata/e{0,1,2}.golden: T4.M should only show up as method of T5 in e2.golden
- reader.go: always include top-level methods, and negate former logic for embedded methods
  (rewrote as a switch for better comprehensability)

Fixes #2791.

R=rsc, rsc
CC=golang-dev
https://golang.org/cl/5576057