cmd/compile/internal/syntax: support for alias declarations

Permits parsing of alias declarations with -newparser

	const/type/var/func T => p.T

but the compiler will reject it with an error. For now this
also accepts

	type T = p.T

so we can experiment with a type-alias only scenario.

- renamed _Arrow token to _Larrow (<-)
- introduced _Rarrow token (=>)
- introduced AliasDecl node
- extended scanner to accept _Rarrow
- extended parser and printer to handle alias declarations

Change-Id: I0170d10a87df8255db9186d466b6fd405228c38e
Reviewed-on: https://go-review.googlesource.com/29355
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/syntax/scanner_test.go

@@ -210,7 +210,8 @@ var sampleTokens = [...]struct {
 	{_IncOp, "--", Sub, precAdd},
 	{_Assign, "=", 0, 0},
 	{_Define, ":=", 0, 0},
-	{_Arrow, "<-", 0, 0},
+	{_Larrow, "<-", 0, 0},
+	{_Rarrow, "=>", 0, 0},
 
 	// delimiters
 	{_Lparen, "(", 0, 0},