My previous strategy here was to try and centralize hspow in one crate,
writing it like a self-contained feature. That introduced friction in
the data types, prompting the use of simplistic types at the netdoc/cell
layers and full-featured types in the optional modules.

This changes tactics, dissolving the low-level parts of tor-hspow into
tor-hscrypto and the high-level parts into hsclient/hsservice. Full
featured types are used everywhere now, but the tradeoff is that
compile-time configurability is a lot more pervasive. Anything that
knows about PoW types at all needs to be fully configured out. I took
this opportunity to try a more complete set of crate features, allowing
users to configure individual PoW schemes.

Co-authored-by: Micah Elizabeth Scott <beth@torproject.org>