Refactor relay cell crypto into a new relaycrypt.c
The functions relay_crypt, relay_crypt_one_payload, and relay_init_cpath_crypto, as well as extractable parts of onionskin_answer and circuit_free_cpath_node -- belong in their own conceptual module. Considered as such, they become fairly easy to test.
We have identified this work as valuable for modularization, for testing, and potentially valuable for testing future relay crypto designs.
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