Trivial fixes

[isislovecruft]

Hi! I was asked to take a look at certain portions of Schnorrkel for a client. I didn't find anything wrong with the parts of the code they are using, but I found a few typos along the way that I've fixed up. I also changed attach_test_vector_rng() to be pub(crate) since it's only used in tests, and as noted in its docs, it's dangerous.

[burdges]

All these changes look good. Thank you! Yes many typeos.