Proposal: bring BBP to Arrays

It looks like Array and IArray have missed some important BBP bits. In particular, we have great ways to build arrays out of lists, but not out of other Foldables. I think we can fix IArray by adding new methods. I think we can fix up Array by adding some functions to Data.Array or by sticking Data.Foldable underneath GHC.Arr and sticking them in there (the low-level fiddling involved would be a better match for GHC.Arr).