Incorrect parsing of hex values

[MikhailRyazanov]

ykparse (and probably everything else) incorrectly parses hexadecimal strings with upper-case letters. Namely, all upper-case letters are treated as zeros instead of treating the letters A–F the same way as a–f (and all other symbols besides 0–9, A–F, a–f as input errors).

Example:

$ ykparse 0123456789ABCDEFGHIJKLMNOPQRSTUV cbdefghijklnrtuvcbdefghijklnrtuv
Input:
  token: cbdefghijklnrtuvcbdefghijklnrtuv
          01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef 
  aeskey: 0123456789ABCDEFGHIJKLMNOPQRSTUV
          01 23 45 67 89 00 00 00 00 00 00 00 00 00 00 00 
...