ptc — guaranteed Proper Tail Calls

[fare]

I had an old project ptc to ensure proper tail calls in CL: https://gitlab.common-lisp.net/frideau/ptc.git

In response to an issue report in scribble (included in QL), I've updated ptc and had scribble use it. But ptc isn't in QL (yet), so I'm hereby for QL to include ptc, so scribble won't break for QL users when QL next tries to update it.