For loop rework chapter 1

[Simn]

This PR changes how we deal with for loop typing by turning it into a two-step approach:

1. Determine the iterator kind and the type of whatever we're iterating over (of_texpr).
2. Use the iterator kind to generate the optimized output code (to_texpr).

We have to determine the type of the thing we're iterating over in order to properly declare the iteration variable, which we need in order to type the loop body. However, we do not necessarily have to do the actual optimization right away and there are cases where delaying it leads to better code generation.

For the time being, we still do step 2 right away. I have already tried the delay and it passes the unit tests, but I'm holding back on this change because I think that it leads to some duplicate typing. Once we get that right, it will address #3663.

What it does right now:

• Unrolling of for (i in 0...5) loops using the same cost calculations as array declaration unrolling does (example: https://i.imgur.com/psfa6qv.png)
• Support for backwards iterators if the bounds are known
• Fix for Abstract iterator #3539

This comes with some temp var name changes, so diffs are a bit messy. Other than that, unit.js looks as expected.