Add support for parallel array initialisation

[kayceesrk]

Array initialisation is likely to be the source of bottlenecks in parallel workloads. Array initialisation for arbitrary typed elements needs to be sequential since the elements need to be initialised to a sensible value before GC gets to see it. However, for primitive types, int and float, one can allow parallel initialisation. OCaml supports Array.create_float, which returns uninitialised array, which can then be initialised in parallel. One can also safely return an uninitialized integer array by

let create_int : int -> int array = Obj.magic (Array.create_float)

Once the array is created, it can then be initialised in parallel.

[kayceesrk]

@sudha247 you might be interested implementing this.

[kayceesrk]

Some work in progress to support this upstream: https://github.com/ocaml/ocaml/pull/9513

[kayceesrk]

Better way to implement this thanks to @stedolan https://gist.github.com/stedolan/f0cb0f5ab24f283d398e48763db1ae62