Remove .pop's "is rw" and fix the fallout from unshift's use of that
fix thinko in Win32 canonpath
Revert "Remove .pop's "is rw" and fix the fallout from unshift's use of that"

This reverts commit 68301a78dc644229fa747250d7119500103f2baa.
Make $a=Nil work
Make Nil work for typed containers, alas not for untyped containers

Suggestions welcome.