* refactor: add SAX-like benc parser
This is the first part of a series of PRs whose end goal is to avoid
the overhead of tr_variant when parsing bencoded data, e.g. when
parsing .torrent files on startup or when parsing announce/scrape
tracker responses.
This PR introduces a SAX-like benc parser, reimplements variant-benc
to use the SAX benc parser (so that we don't have two benc parsers),
and updates the benc + variant tests.
