We perform this operation in a bunch of places, and most of them
use hex::decode(). That's not great, since hex::decode() has to do
heap allocation. This implementation uses hex::decode_to_slice(),
which should be faster.
(In the future we might choose to use one of the faster hex
implementations, but I'm hoping that this change will be sufficient
to get hex decoding out of our profiles.)
Part of #377.
We don't currently need a couple of the key manipulation features
that we have, since we aren't yet doing relays or onion service
clients.
Part of #125