I'll add a high-assurance implementation from Galois to that which is probably not constant time. Their blog and Github has quite a few useful tools.

https://galois.com/blog/2013/09/high-assurance-base64/

Also, anyone wanting constant time implementation might just run a verified implementation through something like FaCT or Jasmin:

https://cseweb.ucsd.edu/~dstefan/pubs/cauligi:2017:fact.pdf

https://acmccs.github.io/papers/p1807-almeidaA.pdf