Add convenient alias to private visibility

Since AccessMap never downgrades visibility, 'private' in AT is
effectively equivalent to 'no change'. Useful in case you want to
change other flags but not the visibility. Accept '*' as private.

src/main/java/net/md_5/specialsource/AccessChange.java

@@ -52,6 +52,7 @@ public class AccessChange {
     static {
         accessCodes.put("public", Opcodes.ACC_PUBLIC);
         accessCodes.put("private", Opcodes.ACC_PRIVATE);
+        accessCodes.put("*", Opcodes.ACC_PRIVATE); // lowest access level (no change)
         accessCodes.put("protected", Opcodes.ACC_PROTECTED);
         accessCodes.put("default", 0);
         accessCodes.put("", 0);