Element-wise arithmetic operations

[superbobry]

It would be nice to have a module, which operates on arrays element-wise:

let open Elementwise in 
abs xs -. ys

However, implementing this requires more thinking:

• Can we do deforrestation of any kind? for instance, operations might return a proxy object, which can only be evaluated with some special function.
• What about scalar-operations, vs /. 42.0?