Optimize for loop over fixed-length sequence

[JukkaL]

If we have a for loop over a fixed-length list or tuple, we can store the iterated values in a stack-allocated array and don't need to allocate a temporary list/tuple object. Example:

for n in [x, y, z]:
    f(n)

If we are iterating over a set, we can't easily do this, since the order of items is unpredictable.

If we are iterating over unboxed types, we can probably avoid boxing/unboxing as well.