Bridge between Scilab and OCaml

It is OK to box pointers as nativeints in the C implementation but we probably don't want to know that they are nativeints from OCaml, or at least that they cannot be forged:
```
type var (* and replace all occurences of nativeint by var*)
```
or at least:
```
type var = private nativeint (* and replace all occurences of nativeint by var*)
```
Also (for later), maybe we could box the current context (or a callback counter) inside variable pointers, so that we can check that they do not escape their their scopes.