std.foldl1, std.foldr1

Fold variants which use first/last element as the starting value of the accumulator.

E.g. `sum(arr):: std.foldl1(function(x, y) x + y, arr)`.

In this case it's not hard to come up with a valid neutral element (0) and use the regular `fold`. But sometimes it can be awkward and/or inefficient.