Added a bounds_info scope to the BranchCollector to help simplify work during the computation of branch bounds. When I encounter a for loop that has a Variable as an extent, I add an interval to the bounds_info that runs from 0 to the variable's type max. This constraint, that extents must be >= 0, allows us to greatly simplify the branch extent exprs. I believe more simplification is possible, but I will leave that for another PR.