The implementation is divided into 4 phases:
1) export filtering of an incoming AST if necessary (exports.go)
2) reading of a possibly filtered AST (reader.go: type reader)
3) method set computation (reader.go)
4) sorting and creation of final documentation (reader.go)

In contrast to the old implementation, the presentation data
(Names, Docs, Decls, etc.) are created immediately upon reading
the respective AST node. Also, all types are collected (embedded
or not) in a uniform way.

Once the entire AST has been processed, all methods and types
have been collected and the method sets for each type can be
computed (phase 3).

To produce the final documentation, the method sets and value
maps are sorted.

There are no API changes. Passes the existing test suite unchanged.

R=rsc, rogpeppe
CC=golang-dev
https://golang.org/cl/5554044