Print \Umathchar in a form that is legal input to it

\show\sumop was printing “\sumop=\Umathchar"202211” which is not legal
input, now it prints “\sumop=\Umathchar"1"0"2211”.

http://tug.org/pipermail/xetex/2013-January/023972.html
Print actual non-BMP chars with \show et al.

Make \show, \meaning, \showlists and logging missing chars print the
actual character for non-BMP ones instead of just printing the hex code.