Expose an order for folding over the context

This MR exposes an additional parameter to the functions folding over the context.

On flattened stores, some directories contain a large number of entries. Folding over such nodes - while ensuring that objects are visited in a sorted order - will require loading the entire node in memory, which is not memory efficient.

The fold functions now expose a parameter order: [Sorted | Undefined], where the Undefined option is memory efficient even when folding over large directories.

For simplicity, I have set the order to Sorted everywhere for now.

Irmin exposes a third option Random of Random.State.t but because Random is not defined in lib_protocol, I did not exposed it in lib_context. But it would be useful to do that for testing purposes: to ensure that, whenever the order is not set to Sorted then indeed the folding order is not relied on.