Compute positions early

[aalexandrov]

At the moment, comprehended term positions are computed late in the compiler pipeline.
This causes erroneous rendering in the web GUI if the comprehended tree is the result of some logical transformation (e.g. fold-group fusion).

To alleviate this problem, I suggest to have a pre-pass on the tree that computes the positions for each comprehended term before any transformations take place.

To implement this feature, the following changes are required:

1. Either in or after Step 1 of the createComprehensionView method, compute the rectangles
   associated with each subtree occurring under term in terms.
2. Based on the inlining relation specified in inlineComprehensions, compute sets of
3. Keep track of the original positions in the ExpressionRoot nodes.
4. Amortize the result for subsequent calls of createComprehensionView.
5. Use the positions set associated with the ExpressionRoot in the expandComprehension method.

[aalexandrov]

This needs to be resolved in the new IR.