Rename structPointer_Map to structPointer_NewAt.

It was never map-specific, and will soon be used for other types.
diff --git a/proto/encode.go b/proto/encode.go
index b46f760..72c780b 100644
--- a/proto/encode.go
+++ b/proto/encode.go
@@ -1084,7 +1084,7 @@
 			repeated MapFieldEntry map_field = N;
 	*/
 
-	v := structPointer_Map(base, p.field, p.mtype).Elem() // map[K]V
+	v := structPointer_NewAt(base, p.field, p.mtype).Elem() // map[K]V
 	if v.Len() == 0 {
 		return nil
 	}
@@ -1123,7 +1123,7 @@
 }
 
 func size_new_map(p *Properties, base structPointer) int {
-	v := structPointer_Map(base, p.field, p.mtype).Elem() // map[K]V
+	v := structPointer_NewAt(base, p.field, p.mtype).Elem() // map[K]V
 
 	keycopy, valcopy, keybase, valbase := mapEncodeScratch(p.mtype)