More idiomatic naming (in particular, matches the naming used for
go/types.Signature).

Also, convert more code to use these methods and/or IterFields.
(Still more to go; only made a quick pass for low hanging fruit.)

Passes toolstash -cmp.

Change-Id: I61831bfb1ec2cd50d4c7efc6062bca4e0dcf267b
Reviewed-on: https://go-review.googlesource.com/20451Reviewed-by: Ian Lance Taylor <iant@golang.org>