Fix #20 (Compile errors with latest rustc)

Fix #20.

- ```ReqParseError``` now derives ```Copy``` to silence a compiler warning.
- All comparison functions on ```Predicate``` now borrow self.
  This was causing errors in ```Predicate.matches()```, where ```self``` was being moved while it was borrowed.
- ```LexState``` now derives ```Copy``` because otherwise it is moved out of its ```Lexer``` in the ```next!``` macro (line 523).
- In ```PredBuilder.build()```, ```PredBuilder.op``` is now cloned if it is not ```None```.