cur_cf is definitely not immutable.
Update to (not very recent) RPython changes.
Reflect movingbuffer API changes.
Making get_slice elidable is a somewhat pointless optimisation.

It's also hard to convince RPython that it's safe (though it is), because on one
path it can raise an exception.
These can all now be fully elided.

If memory serves, at one point an RPython bug prevented this.
Prevent recursive functions from being inlined.
A better metric for recursive functions; also detect functions which contain loops.

We want to inline neither.