Missing a few test cases:
* `foldl` and `foldr` on empty and one element ranges, with and without seeds.
* `foldr1` not tested at all.
* Test ranges with element types other than int (e.g. string concat).
* Test an input range for `foldl`.
* Test a bidir range for `foldr`.
* Test ranges of `const`/`immutable` elements (these always seem to get us).