FoldableWithIndex instance leaking structure

[matthewleon]

Several others:

All FoldableWithIndex folds are leaking structure, as they use toUnfoldable. The cases of foldlWithIndex and foldMapWithIndex are addressed by #137

This leaves foldrWithIndex. It should probably be switched to use toAscUnfoldable, or solved more creatively.

[matthewleon]

I edited the above to remove some of the functions that I thought were leaking structure... Though they used an inefficient strategy for building their output, it was still ordered.