avoid copying the whole map to the stack on each call to get_I8_2_utf()

took run time from 51.6 sec to 41.6 sec

get_I8_2_utf() is called 147000 times by cp_2_utfbytes() which
typically doesn't use the whole table, so return a reference instead to.
avoid copying the while ebcidic mapping to the stack calling get_a2n()

from 41.6sec to 34.1sec

get_a2n() is called 181540 times by __uni_latin1() which in most cases
doesn't use the whole table.  Other callers tend to use the whole
table, so make a copy.
Optimize regen/regcharclass.pl

This became noticably slower recently, these changes don't return us
to the previous runtimes, but they do improve things.