Add _SELF for recursive rules

[xparq]

(Requires #5/2: parent/child tree, for searching...)

It's impossible to deduce implicitly, which exact enclosing rule (level) a standalone (nullary) _SELF should refer to.

So, there should be support in the grammar def. syntax to mark a rule for later reference.

• Currently, there's _DEF/_USE (_NAME, _CALL, _GOTO (and/or: _DEF, _USE, ...) #15), but TBH, that feels more like a workaround than a solution.
• Come up with a light syntax + OPCODE semantics to mark a rule anonymously, for referencing it thereafter implicitly (by _SELF).
  • _SELF could then just mean _USE , and then "just" a solution for a (nullary) position marker would be needed.
  • The problem with that is... actually the same horrible burden, though, as the direct address references of e.g. _CALL_DIRECT, GOTO_DIRECT with auto-link #16...

Also note that named references -- albeit inefficient (full syntax-tree search for every deref.! :-o ) -- allow named rule definitions after a referencing _USE, whereas the implicit marker + _SELF logic couldn't really do that.