[OCaml] Code simplification using option allocation functions

Using the `caml_alloc_some` and `ptr_to_option` functions that
allocate OCaml `option` values enables simplifications in many
cases. These simplifications also result in avoiding unnecessary
double initialization in many cases, so yield a minor optimization as
well.

Also, change to avoid using the old unprefixed functions such as
`alloc_small` and instead use the current `caml_alloc_small`.

A few of the changed functions were slightly rewritten in the
early-return style.

Differential Revision: https://reviews.llvm.org/D99473