Right now there are no library functions for folding over arrays (or
bytevectors, etc.) This implements them. fold-all produces a single value
regardless of the number of dimensions of the array; it is implemented on
top of array-for-each. array-fold produces an array of rank n-1, where n is
the rank of the original array.

I'll write up documentation if the code looks acceptable.

Regards,
Jez