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.