rust-cpython: allow mutation unless leaked reference is borrowed
In other words, mutation is allowed while a Python iterator holding PyLeaked
exists. The iterator will be invalidated instead.
We still need a borrow_count to prevent mutation while leaked data is
dereferenced in Rust world, but most leak_count business is superseded by
the generation counter.
decrease_leak_count(py, true) will be removed soon.
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2