Add fold functions for arrays

At the moment it is quite difficult to perform some folds on some
arrays.

* In general, arrays can be folded over by folding over the range of
  indices and then indexing into the array. This is a little cumbersome,
  and also inefficient when folding right-to-left because Ix does not
  offer reversed range generation.
* Alternately, Array has a Foldable instance which works great, but
  UArray cannot be Foldable. Folds on UArray can instead be done via
  elems. Due to list fusion, this works out well for some folds (foldr,
  foldl'), but not others (foldr').
* For mutable arrays, there are no alternate ways to fold.

This commit adds some commonly used folds for arrays and mutable arrays
to improve this situation.