cosmetic cleanups to the OCaml code

This PR undoes the evil manual-unboxing performed by @lpw25 in #29 , but on my (admittedly x86-64) machine the performance cost is not worth the ugliness.

Ten runs before this PR:

```
8981 LANGUAGE Ocaml 1219
8981 LANGUAGE Ocaml 1232
8981 LANGUAGE Ocaml 1260
8981 LANGUAGE Ocaml 1244
8981 LANGUAGE Ocaml 1239
8981 LANGUAGE Ocaml 1330
8981 LANGUAGE Ocaml 1242
8981 LANGUAGE Ocaml 1249
8981 LANGUAGE Ocaml 1250
8981 LANGUAGE Ocaml 1254
```

Ten runs after this PR:

```
8981 LANGUAGE Ocaml 1275
8981 LANGUAGE Ocaml 1290
8981 LANGUAGE Ocaml 1279
8981 LANGUAGE Ocaml 1280
8981 LANGUAGE Ocaml 1280
8981 LANGUAGE Ocaml 1277
8981 LANGUAGE Ocaml 1275
8981 LANGUAGE Ocaml 1286
8981 LANGUAGE Ocaml 1277
8981 LANGUAGE Ocaml 1275
```