Implement iterators that mirror Stdlib.Array

[kayceesrk]

It would be useful to implement parallel versions of Stdlib.Array iterators (see https://v2.ocaml.org/api/Array.html#1_Iterators). In particular, implement iter, iteri, map, map_inplace, mapi, mapi_inplace.

Related to

• Question: Does a parallel map function exist in Domainslib? #117
• parallel_map #37