_DEF/_USE, recursion etc. (#14, #15)

Nested structures like { some = code; { block; {another} stuff} yay }
can be parsed now recursively.

+ Rule tree: name lookup for _USE (no generic find() yet)
+ #14: Recursion is implemented, but not via _SELF yet
+ #15 (_DEF/_USE)
+ #28: Parser::run() added
+ OPERATORS -> CONST_OPERATORS, preparing to taking it seriously
  (RULE.name is mutable though, for _DEF to still work on const rules,
  but that's a tmp. kludge!)
+ Fix #29: Crash in _DEF for missing len = 0
+ Better diagnostics (tests, debug messages etc.)
+ Comments, cosmetics