Add Atomic_array

[polytypic]

This PR adds an Atomic_array module that gives a limited functionality equivalent of 'a Atomic.t array using internally only an 'a array based on the caml_atomic_cas_field function that already exists in the OCaml 5.0.0 - 5.2.0 runtime.

The benefit of this is that only 1 word, instead of 3 words, is used per array element (i.e. a significant drop in memory use and allocations) and an indirection is avoided (i.e. less pointer chasing). This translates to, depending on the case, to significant performance improvements in the implementation of array based lock-free data structures (up to 40% improvement has been seen in some cases).