Support for SHAKE256, based on patch by Daiki Ueno.

* shake256.c (sha3_256_shake): New file and function.
* Makefile.in (nettle_SOURCES): Add shake256.c.
* testsuite/testutils.c (test_hash): Allow arbitrary digest size,
if hash->digest_size == 0.
* testsuite/shake.awk: New script to extract test vectors.
* testsuite/Makefile.in (TS_NETTLE_SOURCES): Add shake256-test.c.
(DISTFILES): Add shake.awk.