go/ast/astutil: add Apply, for rewriting Go ASTs

This change implements an AST walker, Apply, with the capability
to perform various actions (including rewrites) during the AST
walk, in pre- and post-iteration order.

Ideally we would like to have this functionality in the std lib
(go/ast), but as this change contains significant new API, it
is better to first gain some experience with it in x/tools.

Credits: This is joint work of Josh Bleecher Snyder, Roger
Peppe, and myself (author of this CL). The code is based on
proposal golang/go#17108 which I had started, together with an
initial implementation. It was then reworked significantly by
Josh in golang.org/cl/55790, further refined by suggestions
from Roger (allocation reduction), and finally cleaned up a bit
by me (AST simplifications).

Fixes golang/go#17108.

Change-Id: I56d56b7f2bc5be2acfeb927f76aea7f264bb7b94
Reviewed-on: https://go-review.googlesource.com/77811
Run-TryBot: Robert Griesemer <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: griesemer