cipher: Add Streamlined NTRU Prime sntrup761.

* cipher/Makefile.am (libcipher_la_SOURCES): Add sntrup761.c and h.
* cipher/sntrup761.c: New.
* tests/Makefile.am (tests_bin): Add t-kem.
* tests/t-kem.c: New.

Verbatimly apply a part of changes from
https://gitlab.com/jas/libgcrypt in jas/sntrup761v2 branch.

• Signed-off-by: Simon Josefsson <simon@josefsson.org>