Implement standard Rust crypto traits

[macpijan]

Currently, Trussed doesn't implement any of the traits which have become standard in Rust ecosystem, such as rand_core::Rng, digest::Digest to name a few. If these were implemented, it would be easier to use the known APIs, and allow for greater portability, as Trussed could be used as one of the possible crypto backends for certain application.

This has also be discussed here.

Note that our knowledge of the current state of Trussed could be a bit of out of date, so please feel free to correct us if we are wrong.