Forbid above IV_MAX code points

Forbid above IV_MAX code points

This implements the restriction of code points to 0..IV_MAX in such a
way that the process doesn't die when presented with input UTF-8 that
evaluates to a larger one.  Instead, it is treated as overflow.

The commit reinstates causing the offending process to die if trying to
create a character somehow that is above IV_MAX (like
chr(0xFFFFFFFFFFFFF) or trying to do certain operations on one if
somehow one did get created.

The long term goal is to use code points above IV_MAX internally, as
Perl6 does.  So code and tests are not removed, just commented out