go/types: minor cleanups

go/types: minor cleanups

1) Removed mark field from declInfo struct. Instead use a visited map
   in ordering.go which was the only use place for the mark field.

2) Introduced objSet type for the common map[Object]bool type.

3) Improved comments.

Change-Id: I7544e7458d844b0ca08193f11de6238d317eaf2d
Reviewed-on: https://go-review.googlesource.com/24153
Reviewed-by: Alan Donovan <adonovan@google.com>