encoded length macros for baseXX encodings
It can be useful to have macros that give the lengths of byte strings encoded with baseXX, particularly for declaring fixed-size buffers. Some use cases will need to include a terminating NUL, others not. These macros won't check for overflow, so nothing should pass untrusted inputs to them.
I propose macros named like
BASE64_LEN() for the raw length and like
BASE64_SIZE() for the length with NUL.