We want to use the circuit_init_cpath_crypto() function to setup our
cpath, and that function accepts a key array as input. So let's make our
HS ntor key expansion function also return a key array as output,
instead of a struct.
Also, we actually don't need KH from the key expansion, so the key
expansion output can be one DIGEST256_LEN shorter. See here for more
info: https://trac.torproject.org/projects/tor/ticket/22052#comment:3
