reflect: add Value.Bytes, Value.SetBytes methods

This allows code that wants to handle
[]byte separately to get at the actual slice
instead of just at individual bytes.
It seems to come up often enough.

R=r
CC=golang-dev
https://golang.org/cl/4942051

src/pkg/reflect/value.go

@@ -398,6 +398,18 @@ func (v Value) Bool() bool {
 	return *(*bool)(unsafe.Pointer(&iv.word))
 }
 
+// Bytes returns v's underlying value.
+// It panics if v's underlying value is not a slice of bytes.
+func (v Value) Bytes() []byte {
+	iv := v.internal()
+	iv.mustBe(Slice)
+	typ := iv.typ.toType()
+	if typ.Elem().Kind() != Uint8 {
+		panic("reflect.Value.Bytes of non-byte slice")
+	}
+	return *(*[]byte)(iv.addr)
+}
+
 // CanAddr returns true if the value's address can be obtained with Addr.
 // Such values are called addressable.  A value is addressable if it is
 // an element of a slice, an element of an addressable array,
@@ -1224,6 +1236,19 @@ func (v Value) SetBool(x bool) {
 	*(*bool)(iv.addr) = x
 }
 
+// SetBytes sets v's underlying value.
+// It panics if v's underlying value is not a slice of bytes.
+func (v Value) SetBytes(x []byte) {
+	iv := v.internal()
+	iv.mustBeAssignable()
+	iv.mustBe(Slice)
+	typ := iv.typ.toType()
+	if typ.Elem().Kind() != Uint8 {
+		panic("reflect.Value.SetBytes of non-byte slice")
+	}
+	*(*[]byte)(iv.addr) = x
+}
+
 // SetComplex sets v's underlying value to x.
 // It panics if v's Kind is not Complex64 or Complex128, or if CanSet() is false.
 func (v Value) SetComplex(x complex128) {