go/types, types2: fix tracing output for type and expr lists

go/types, types2: fix tracing output for type and expr lists

- support printing of expression and type lists in sprintf
- simplified some code in go/types/exprstring.go
- fixed a typo in syntax package

Change-Id: Ic4bc154200aad95958d5bc2904a9ea17cf518388
Reviewed-on: https://go-review.googlesource.com/c/go/+/377974
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>