reflect: add Type.ConvertibleTo, Value.Convert (API CHANGE)

Fixes #4047.

R=iant, r
CC=golang-dev
https://golang.org/cl/6500065

src/pkg/reflect/export_test.go

@@ -0,0 +1,16 @@
+// Copyright 2012 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.
+
+package reflect
+
+// MakeRO returns a copy of v with the read-only flag set.
+func MakeRO(v Value) Value {
+	v.flag |= flagRO
+	return v
+}
+
+// IsRO reports whether v's read-only flag is set.
+func IsRO(v Value) bool {
+	return v.flag&flagRO != 0
+}