Now that HMAC isn't special anymore, and with the help of init params,
this isn't needed anymore as we only allocate memory for the state.
This effectively reverts e9bcc64e62 .
Now that there are dedicated struct for initial parameters, make the
HMAC params provide a lc_hash_ctx to be used internally. This makes HMAC
less special and allows it to work with any hash, instead of needing
dedicated inits and ctx_news. As an upside, now it's possible to have a
one-pass HMAC.
