[dev.typealias] go/ast, go/parser, go/printer, go/types: initial type alias support

Parsing and printing support for type aliases complete.
go/types recognizes them an issues an "unimplemented" error for now.

For #18130.

Change-Id: I9f2f7b1971b527276b698d9347bcd094ef0012ee
Reviewed-on: https://go-review.googlesource.com/34986


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