jit_ocaml: ScilabInterp executes sum1.sci using Scilab prims

Thanks to many stubs in ScilabInternalType.ml allowing to access
Scilab values from OCaml, ScilabInterp is able to execute very simple
Scilab programs, at almost the same speed as Scilab.  It uses the new
implementation of contexts, that are not available on Scilab side
(i.e. Scilab gateways modifications to the context are not seen from
OCaml). Scilab functions can call macros generated by the OCaml
interpreter, using the new OCamlFunction C++ type (not yet tested).