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

Python2.7 uses protocol 1 by default, which is not compatible with
class defining __slots__. On the other hand, all the Python versions
support versions >= 2.

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

Python2.7 uses protocol 1 by default, which is not compatible with
class defining __slots__. On the other hand, all the Python versions
support versions >= 2.

- add methods for visiting intermediate nodes
- add method to notify subclass of cycles
- support returning iterables of nodes from visit*node_in functions
- remove result and visit return value
- also add children property to PackedNode

fixes #663

The result is a much smaller file.