Advisories for Cargo/Sodiumoxide package

The scalarmult() function included in previous versions of this crate accepted all-zero public keys, for which the resulting Diffie-Hellman shared secret will always be zero regardless of the private key used. This issue was fixed by checking for this class of keys and rejecting them if they are used.

An issue was discovered in the sodiumoxide crate starting with 0.2.0 and prior to 0.2.5 for Rust. generichash::Digest::eq compares itself to itself and thus has degenerate security properties.