This patch implements Salsa20, an eSTREAM candidate. This patch should
be applied after the "stream" patch.
Notable differences from my first posting (25 Oct):
1. Smaller test vectors are used. Instead of 512 bytes, now I use 38,
64, 111 and 129 instead. Reason: (a.) to test a variety of lengths
that are not necessarily multiple of the block size; (b.) to reduce
the size of the patch.
2. salsa20_encrypt_bytes() becomes salsa20_encrypt_block() to match
the changes in stream.c.
The test vectors agreed. Also I had successes with "cryptsetup
luksFormat -c salsa20-stream-plain /dev/loop0".