poly1305: implement a subset of the hash.Hash interface

This CL adds the poly1305.MAC type which implements a
subset of the hash.Hash interface. With MAC it is possible
to compute an authentication tag of data without copying
it into a single byte slice.

This commit modifies the reference/generic and the
AMD64 assembler but not the ARM/s390x implementation
to support an io.Writer interface.

Updates golang/go#25219

Change-Id: I7ee5a9eadd43387cf3cd887d734c625575eee47d
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/111335
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

poly1305/mac_noasm.go

@@ -0,0 +1,11 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !amd64 gccgo appengine
+
+package poly1305
+
+type mac struct{ macGeneric }
+
+func newMAC(key *[32]byte) mac { return mac{newMACGeneric(key)} }