- change Walk signature to use an ast.Node instead of interface{}
- add Pos functions to a couple of ast types to make them proper nodes
- explicit nil checks where a node can be nil; incorrect ASTs cause Walk to crash

For now ast.Walk is exercised extensively as part of godoc's indexer;
so we have some confidence in its correctness. But this needs a test,
eventually.

Fixes #1326.

R=rsc, r
CC=golang-dev
https://golang.org/cl/3481043