Make case configurable in base16 encoder

[magthe]

I notice that your base16 encoder outputs upper-case (A--F), while some of the other Haskell base16 encoders go with lower-case (a--f). None appear configurable. It might be nice to have a variant of base16 for each choice of case, so that no post-processing is required, in case it matters (even cosmetic changes can break existing tests in some end-user code).

Originally posted by @vdukhovni in #16 (comment)