There are a number of alphabets in common use for base32
and base64 encoding, such as url-safe encodings.
This adds support for passing a function to encode into
arbitary alphabets.
the string encoding functions touch secret key material
in a bunch of places (devtls, devcap), so make sure we do
not leak information by cache timing side channels, making
the encoding and decoding routines constant time.
we also expose the alphabets through encXchr()/decXchr()
functions so caller can find the end of a encoded string
before calling decode function (for libmp).
the base32 encoding was broken in several ways. inputs
lengths of len%5 == [2,3,4] had output truncated and
it was using non-standard alphabet. documenting the alphabet
change in the manpage.
