[OCaml] Fix unsafe uses of Store_field

Using `Store_field` to initialize fields of blocks allocated with
`caml_alloc_small` is unsafe. The fields of blocks allocated by
`caml_alloc_small` are not initialized, and `Store_field` calls the
OCaml GC write barrier. If the uninitialized value of a field happens
to point into the OCaml heap, then it will e.g. be added to a conflict
set or followed and have what the GC thinks are color bits
changed. This leads to crashes or memory corruption.

This diff fixes a few (I think all) instances of this problem. Some of
these are creating option values. OCaml 4.12 has a dedicated
`caml_alloc_some` function for this, so this diff adds a compatible
function with a version check to avoid conflict. With that, macros for
accessing option values are also added.

Differential Revision: https://reviews.llvm.org/D99471