[PATCH] Add A5 and GEA ciphers

Hello.

Attached is A5/3,4 GEA3,4 implementation which was described today at OsmoDevCon.

This is "from the scratch" implementation, tests using test vectors from relevant 3GPP standards are included.

There are couple of bits missing: * gprs_auth API needs to be changed to use GEA4 with 128 bits key * gea_test only checks algorithm correctness but not loading/using it via gprs_auth plugin api * osmo_a5_1 and osmo_a5_2 probably should be hidden from public api - the only way to call them should be through osmo_a5() function

Please review and merge if possible.