* sha512.c: Reorganized to use _nettle_sha512_compress.

* sha512-compress.c (_nettle_sha512_compress): Compression
function extracted from sha512.c to a new file.

* Makefile.in (nettle_SOURCES): Added sha256-compress.c and
sha512-compress.c.

Rev: nettle/ChangeLog:1.57
Rev: nettle/Makefile.in:1.18
Rev: nettle/sha.h:1.4
Rev: nettle/sha512-compress.c:1.1
Rev: nettle/sha512.c:1.2