Support lowercase bases?

[luizirber]

This error in rust-mdbg is triggered because lowercase bases are passed to nthash. Possible solutions: 1) users of the crate need to provide uppercase (non-repeat masked) nucleotides. This will probably trigger another allocation, so potentially slower. 2) accept lowercase bases in nthash. One solution is modifying H_LOOKUP and RC_LOOKUP to also have the lowercase variants.

• should this also have some sort of "strict mode" that doesn't accept lowercase bases, declared in the constructor?

[RagnarGrootKoerkamp]

Similarly, it may be nice to accept [0,1,2,3] values for convenience as well, for when data is already packed.