go/ast, go/parser: parse alias declarations

go/ast, go/parser: parse alias declarations

For now, we also accept "type p = p.T" (using = instead of =>, for
type aliases only), so we can experiment with an approach that only
uses type aliases. This concession is implemened in the parser.

For #16339

Change-Id: I88b5522a8b6cfc2e97ca146ede8b32af340220f8
Reviewed-on: https://go-review.googlesource.com/30211
Reviewed-by: Matthew Dempsky <mdempsky@google.com>