Old version of rand_core

[meh]

Is there a reason an older version of rand_core is being used?

[tcharding]

This is because we use rand version 0.6 which is explained in #14 and over in rust-bitcoin/rust-secp256k1#328

[elsirion]

Not sure if this is still actionable. Seems the version the complaint was about got upgraded (we now support rand 0.8), but there's newer versions to upgrade to which would require a rust-bitcoin ecosystem-wide upgrade.