handy.h: Convert XDIGIT_VALUE to branchless

handy.h: Convert XDIGIT_VALUE to branchless

This removes a branch and an array lookup in the XDIGIT_VALUE() macro.
It adds some shifts, masks, and additions instead, though replacing a
mask and addition in the old way.

A somewhat more complicated version could be made for EBCDIC, but I'm
not bothering to do that, using an array lookup to convert the salient
value to ASCII, so on EBCDIC there isn't an array lookup removal.