[dev.typeparams] cmd/compile/internal/syntax: clean up node printing API

Preparation for using the syntax printer as expression printer in types2.

- Introduced Form to control printing format
- Cleaned up/added String and ShortString convenience functions
- Implemented ShortForm format which prints … for non-empty
  function and composite literal bodies
- Added test to check write error handling

Change-Id: Ie86e46d766fb60fcf07ef643c7788b2ef440ffa8
Reviewed-on: https://go-review.googlesource.com/c/go/+/282552


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