Remove use of unsafe

[psivesely]

With upcoming changes in sodiumoxide (sodiumoxide/sodiumoxide#208 and sodiumoxide/sodiumoxide#196), it should be possible to remove use of unsafe from this library.

After these are merged I believe I will actually have to write a follow-up PR that allows direct calls to crypto_generichash where the out argument is &mut [u8]. This will be much more efficient than dealing with the State object defined in https://github.com/sodiumoxide/sodiumoxide/pull/196/files#diff--ca72970d1aef31492c489302609ede1fR30 or any all-in-a-single-call hashing function that doesn't take an out argument (as I can't think of any way of super-efficiently combining the arrays that doesn't include the use of unsafe, which is what we're trying to get rid of in the first place 😺).