Sodiumoxide Versions Save

Additions:

• Added unit tests & test vectors from libsodium, commit
• Add crypto_kx_* functions to libsodium-sys (#189)

Fixes:

• fix std libraries being pulled in even in no_std environments (#214)

Changes:

• Use tuple-struct indexing (#217)
• Change to rust-bindgen (#208)
• API Improvements (#185)
• Make AEAD Nonce type a nonce new_type (#183)

Deletions:

• libsodium-sys/lib.rs removed, we now use bindgen (#218)