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_field.go b/internal/impl/message_field.go
index 75cf867..e92a7ca 100644
--- a/internal/impl/message_field.go
+++ b/internal/impl/message_field.go
@@ -119,7 +119,7 @@
 		},
 		set: func(p pointer, v pref.Value) {
 			rv := p.apply(fieldOffset).asType(fs.Type).Elem()
-			rv.Set(reflect.ValueOf(v.Map().(pvalue.Unwrapper).Unwrap()).Elem())
+			rv.Set(reflect.ValueOf(v.Map().(pvalue.Unwrapper).ProtoUnwrap()).Elem())
 		},
 		clear: func(p pointer) {
 			rv := p.apply(fieldOffset).asType(fs.Type).Elem()
@@ -151,7 +151,7 @@
 		},
 		set: func(p pointer, v pref.Value) {
 			rv := p.apply(fieldOffset).asType(fs.Type).Elem()
-			rv.Set(reflect.ValueOf(v.List().(pvalue.Unwrapper).Unwrap()).Elem())
+			rv.Set(reflect.ValueOf(v.List().(pvalue.Unwrapper).ProtoUnwrap()).Elem())
 		},
 		clear: func(p pointer) {
 			rv := p.apply(fieldOffset).asType(fs.Type).Elem()