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.go

@@ -229,7 +229,7 @@ redo:
 			goto assignop
 		}
 		if c == '-' {
-			s.tok = _Arrow
+			s.tok = _Larrow
 			break
 		}
 		s.ungetr()
@@ -253,11 +253,16 @@ redo:
 		s.tok = _Operator
 
 	case '=':
-		if s.getr() == '=' {
+		c = s.getr()
+		if c == '=' {
 			s.op, s.prec = Eql, precCmp
 			s.tok = _Operator
 			break
 		}
+		if c == '>' {
+			s.tok = _Rarrow
+			break
+		}
 		s.ungetr()
 		s.tok = _Assign