lsh_string and an index for source and destination. Also changed
the order of source and destination arguments.
(MAC_DIGEST): Take a lsh_string and index for the destination.

* src/crypto.c: Rewrote cipher classes to use functions in
lsh_string.c, instead of writing directly into the output string.
(hash_digest_string): New function.
(hash_digest): New function.
(do_hmac_digest): Use lsh_string_write_hmac.

Rev: src/abstract_crypto.c:1.24
Rev: src/abstract_crypto.h:1.43
Rev: src/crypto.c:1.36
Rev: src/crypto.h:1.34