Generator: use `size_t` rather than `int` in `nlaSolve()`

[agarny]

The external nlaSolve() method currently has the following signature:

extern void nlaSolve(void (*objectiveFunction)(double *, double *, void *),
                     double *u, int n, void *data);

However, rather than using int, we really ought to use size_t as we already do in createVariablesArray() for instance.