Make new functions version aware and support multiple libsodium versions

[mgregoro]

Version detection has already been implemented see c7ab096, just have to add newer functionality and make use of the Crypt_Sodium_XS_sufficient_libsodium_version guard function. Make sure this is done for the xchacha20poly1305 functions before closing #9

crypto_x features introduced in:

Base Support: 1.0.8
ARGON2: 1.0.9
aead_xchacha20poly1305: 1.0.12
Newest: 1.0.16

Allow build against 1.0.8

[mgregoro]

Other commits that implemented version detection using pkg-config and checks for Alien::Sodium 90be0dd, bca1aae