install useful Regexp::CARP_TRACE from Carp

Regexp is a built-in class for which no module is normally loaded, so it
can't provide its own CARP_TRACE method.  Carp must therefore supply it.
The method formats a regexp reference as a qr() expression as much as
possible.  Like string arg formatting, it uses \x{} escapes for literal
characters that are not ASCII printable, and it truncates according
to $Carp::MaxArgLen.  The truncation happens at a different stage of
processing from its position in string arg formatting, because regexp
stringification presents an already-partly-escaped form of the regexp.