Support Nix base32 alphabet

[lf-]

// omitted: E O U T
const std::string base32Chars = "0123456789abcdfghijklmnpqrsvwxyz";

This is a special base32 alphabet that was invented by Eelco Dolstra that the nix ecosystem is going to be stuck with ~forever; originally documented in https://edolstra.github.io/pubs/phd-thesis.pdf.

It would be helpful to have support for it for writing Nix related tooling without needing to write new base32 implementations.