[dev.typeparams] go/printer: adapt changes from dev.go2go

Import go/printer changes from the dev.go2go branch, with the following
modifications:
 - update tests to only use bracketed notation for type parameters
 - remove the UseBrackets mode, since it is now implied
 - remove guards on ast.Field.Type != nil

Patchset #1 contains the dev.go2go source, unmodified except to resolve
merge conflicts.

Change-Id: I3ddecfd3bee0fc32425a30fe6bd93b24fd3187e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/273226


Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>