Additionally, this change extracts the functions that created and
freed these elements.
These structures had common "forward&reverse stream&digest"
elements, but they were initialized and freed through cpath objects,
and different parts of the code depended on them. Now all that code
is extacted, and kept in relay_crypto.c
This should help us improve modularity, and should also make it
easier for people to experiment with other relay crypto strategies
down the road.
This commit is pure function movement.
