- `user_repr` is now a method
- Fix for python 2.7
- excepts -> expected

Note: This refactor opens the door for implementing a ContextualLexer for Earley.
But unlike the existing one for LALR, it will have to be computed at runtime,
rather than ahead of time.

rename LexerConf.terminals to LexerConf.tokens
Make Exception message generation lazy
Made a few classes new-style

The debug flag is already saved in the cached file, but is not
resumed on loading the dumped file. To solve that,
 - add argument `debug` to parser_fronteds.WithLexer.deserialize
 - add argument `debug` lalr_parser.LALR_Parser.deserialize
 - propagate the value of the `debug` option on resuming a cached
   grammar, in lark.Lark._load

The debug flag is already saved in the cached file, but is not
resumed on loading the dumped file. To solve that,
 - add argument `debug` to parser_fronteds.WithLexer.deserialize
 - add argument `debug` lalr_parser.LALR_Parser.deserialize
 - propagate the value of the `debug` option on resuming a cached
   grammar, in lark.Lark._load