simplify swapRemove

[Fri3dNstuff]

The existing implementation needlessly special-cased removal of the last item, even though it is handled perfectly fine by the rest of the code.

Additionally:

• added the documented but non-existent assert that "index is in bounds",
• removed the documented assertion that "list is not empty", as that is already implied by the previous assertion.

If my usage of defer or orelse unreachable is confusing, I'll happily change them to a three-line solution, and .?.

The correctness of this simplification relies on the left-to-right evaluation order of the operands of assignment.

[andrewrk]

This implementation is now equivalent to the previous one, but it can still be better, by setting the invalidated element to undefined, helping to catch bugs, and sometimes even facilitate optimizations.

[Fri3dNstuff]

I've added code to override the removed item with undefined, both in swapRemove and pop.

[andrewrk]

Good work, thank you.