Fix use of incorrect macro in XS (RT#67289)

This line uses the wrong macro to remove magic flag(s) and instead
turns off overloading.  Use the correct macro.
Replace weak set cleanup code (RT#69967)

I didn't know about sv_unmagic when I wrote this magic cleanup
code.  Use it: it's safer and doesn't leak.  The new test script from
mp@leasingborsen.dk is maintainer only (in MANIFEST.SKIP) because it
needs Moose, and this is a low–dependency module.
Changelog for 1.29 release