[dev.typeparams] cmd/compile/internal/syntax: implement parsing of type parameters

Port from dev.go2go prototype branch. The compiler doesn't yet set the
syntax.AllowGenerics mode, so parsing of generic code remains disabled.

Known issue: The doc strings documenting the specific syntax accepted
by parser methods are not all up-to-date.

Change-Id: I13d134289fd9330fd0ed7f97c997cca6f23466fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/261658


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>