Remove base64_decode_nopad() because it is redundant now that
base64_decode() correctly handles both padded and unpadded base64
encodings with "right-sized" output buffers.
The base64 and base32 functions used to be in crypto.c;
crypto_format.h had no header; some general-purpose functions were in
crypto_curve25519.c.
This patch makes a {crypto,util}_format.[ch], and puts more functions
there. Small modules are beautiful!
