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.

make _all_terminals a dict
added raw attribute to Pattern
rename nice_print -> user_repr

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

- Refactored lexer interface into LexerConf
- Lexer now compiles regexps only when used (especially useful for ContextualLexer)
- Lexer now doesn't validate on deserialize (noticable speedup)

- Makes rule ordering the default ambiguity tie breaker.
    E.g.
    start: a | b
    a: "A"
    b: "A"

    will return:
    start
        a

    start: b | a
    a: "A"
    b: "A"

    will return
    start
        b

- Replaces the ambiguity='resolve__antiscore_sum' with a separate option: 'priority'.
    The priority option has 4 values: 'auto', 'none', 'normal', 'invert'.
    'Auto' maps to 'Normal' for CYK and Earley and 'None' for LALR.
    'None' filters your priorities and ignores them. This saves some extra tree walking on Earley.
    'Normal' uses your priorities untouched, mimicing the old behaviour.
    'Invert' negates your priorities, emulating the old 'resolve__antiscore_sum' behaviour.

    This allows you to use priority logic even when ambiguity=='explicit', to get a better idea
    of the shape of your tree; and to easily disable priorities without removing them from the
    grammar for testing (or performance).

- ambiguity='explicit' now correctly returns an ambiguous tree again, as 0.6 did.

types of ambiguity.
 - Rewritten along the lines of Elizabeth Scott's parser.
https://www.sciencedirect.com/science/article/pii/S1571066108001497

 - Implement SPPF trees per Elizabeth Scott and Bram van der Sanden's work.
http://www.bramvandersanden.com/post/2014/06/shared-packed-parse-forest/