internal/impl: unexport Unwrapper

This interface is only ever used within the impl package.
Unexport it to avoid potential abuse by users.

Change-Id: I58d0cc2e8cb01c8c9d36cdec469c8d2196c4f836
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/193197
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/message_reflect.go b/internal/impl/message_reflect.go
index 469b255..b34e479 100644
--- a/internal/impl/message_reflect.go
+++ b/internal/impl/message_reflect.go
@@ -221,7 +221,7 @@
 
 var (
 	_ pref.Message = (*messageState)(nil)
-	_ Unwrapper    = (*messageState)(nil)
+	_ unwrapper    = (*messageState)(nil)
 )
 
 // messageDataType is a tuple of a pointer to the message data and
@@ -240,9 +240,9 @@
 
 var (
 	_ pref.Message      = (*messageReflectWrapper)(nil)
-	_ Unwrapper         = (*messageReflectWrapper)(nil)
+	_ unwrapper         = (*messageReflectWrapper)(nil)
 	_ pref.ProtoMessage = (*messageIfaceWrapper)(nil)
-	_ Unwrapper         = (*messageIfaceWrapper)(nil)
+	_ unwrapper         = (*messageIfaceWrapper)(nil)
 )
 
 // MessageOf returns a reflective view over a message. The input must be a
@@ -266,7 +266,7 @@
 func (m *messageIfaceWrapper) ProtoReflect() pref.Message {
 	return (*messageReflectWrapper)(m)
 }
-func (m *messageIfaceWrapper) ProtoUnwrap() interface{} {
+func (m *messageIfaceWrapper) protoUnwrap() interface{} {
 	return m.p.AsIfaceOf(m.mi.GoReflectType.Elem())
 }