internal/value: rename the Unwrap method as ProtoUnwrap

Add a Proto prefix before the Unwrap method to reduce the probability that
it would ever conflict with a method of the same name that a
custom implementation of Enum, Message, List, or Map may have.

Change-Id: I628bf8335583f2747ab4589f3e6ff82e4501ce98
Reviewed-on: https://go-review.googlesource.com/c/151817
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/impl/message_test.go b/internal/impl/message_test.go
index 6d82456..eeb4e7a 100644
--- a/internal/impl/message_test.go
+++ b/internal/impl/message_test.go
@@ -1124,7 +1124,7 @@
 		return pv.Interface()
 	}),
 	cmp.Transformer("UnwrapGeneric", func(x pvalue.Unwrapper) interface{} {
-		return x.Unwrap()
+		return x.ProtoUnwrap()
 	}),
 	cmpopts.EquateNaNs(),
 	cmpopts.EquateEmpty(),