Implement the 'qr' overload type.

Implement the 'qr' overload type.

If this is defined, it will be called instead of stringification
whenever an object is used as a regexp or interpolated into a regexp.
This will fall back to stringification even without C<fallback => 1>,
for compatibility.

An overloaded 'qr' must return either a REGEXP or a ref to a REGEXP
(such as created by qr//). Any further overloading on the return value
will be ignored.