Changes:
* Allow optimizing partial_apply capturing opened existential: we didn't do this originally because it was complicated to insert the required alloc/dealloc_stack instructions at the right places. Now we have the StackNesting utility, which makes this easier.
* Support indirect-in parameters. Not super important, but why not? It's also easy to do with the StackNesting utility.
* Share code between dead closure elimination and the apply(partial_apply) optimization. It's a bit of refactoring and allowed to eliminate some code which is not used anymore.
* Fix an ownership problem: We inserted copies of partial_apply arguments _after_ the partial_apply (which consumes the arguments).
* When replacing an apply(partial_apply) -> apply and the partial_apply becomes dead, avoid inserting copies of the arguments twice.
These changes don't have any immediate effect on our current benchmarks, but will allow eliminating curry thunks for existentials.
