Fixes.
Helpers.
Updated TODO.
`reaver::parser`: `rule<>` a.k.a. `rule<void>`.
`reaver::callbacks`, based on idea of klmr's `multifunction`.

It's not a final functionality yet; I'll probably add some more member
utility functions to this later; focus for now is to add callback
support to `reaver::parser`.
An intro to `lexer` overhaul, a helper for `parser` callbacks.

Damn, all the changes to the lexer will be pain in the ass, but it
should start having some decent performance at some point (especially
when it'll become possible to avoid calling lexer and keep all the data
in the memory all the time...).
Initial implementation of `lexer::basic_iterator`. Fails hard for some reason.
Fixes. Seems to be working.
Update TODO.