Performance benchmark

[tvercaut]

Many thanks for your useful library.

I really like the structure, testing and documentation that comes with it. I however tested its performance against a few other implementations and found it to be rather slow in comparison to best in class ones: https://github.com/gaspardpetit/base64/pull/11

I understand performance may not be the main target for this library but it may be helpful to mention the banchmark results in the README.

On a related note, seeing the performance of modp_b64, I adapted their approach and created this using some c++20 features: https://github.com/tvercaut/base64/blob/modpb64xover/include/base64.hpp I would benefit from a nicer API and maintenance perks but I thought it could be relevant to share.