Anonymous tokens would become visible if they had the same value as named tokens.
That's because they are merged for the lexer. But after this change, the rules for
visibility are based on their use in the rule, and not their name or identity.

* Removed the scanless parsing feature (dynamic lexing is king)
* Default LALR lexer is now contextual

- @erezsh added the new Lark reference and some other things using the
  current .g extension, so I wanted to resolve them for PR #117
- Renamed lark.g to lark.lark.
- Changed lark_grammar.py to use .lark file extensions.
- Changed urls that used .g to use .lark.

- Merging updated upstream into branch for file extension changes.
- Will push so Pull Request has no remaining conflicts.
- Also will change the file type of lark example grammar.

(inspired by issue #116)

Interpreter class: fixed default behavior to return results

It emulates antlr's visitor behavior for a dynamic evaluation order of subtrees

Fix order of members when pickling Token

I found this while porting Token to C, essentially the value and pos_in_stream members of Token were swapped in ``__reduce__``, which means running ``pickle.loads`` and ``pickle.dumps`` would result in unpickled tokens whose value was the original's position in stream, and vice versa. In my C extension this caused a TypeError exception, but the behavior will have to be corrected in both.

Mostly improvements to memory consumption.

Added __slots__ to RulePtr and Token, resulting in significantly lower memory consumption.

As suggested by @drslump.

- Reasoning addressed in Issue #116
- Renamed example and grammars file to use new extension.
- Changed `.g` to new extension of `.lrk` in places where it is
  referenced.

Ensure Tokens can be pickled correctly