* twofish.c: Adapted to nettle. Made constant tables const.

Deleted bytes_to_word and word_to_bytes; use LE_READ_UINT32 and
LE_WRITE_UINT32 instead.
(twofish_selftest): Deleted. Moved the tests to the external
testsuite.
(twofish_set_key): Don't silently truncate too large keys.

Rev: src/nettle/twofish.c:1.2