[dev.typeparams] cmd/compile/internal/syntax: add Walk node vistor from types2

This moves the Walk visitor from the types2 to the syntax
package. There are no changes but for package name adjustments.
Preparation for a more full-fledged node visitor.

Change-Id: I95217e27ff943ac58a7638fb8d1cd347d0d554b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/282556


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>