`__restrict__` everything

[frostedoyster]

As mentioned in #7, it might be beneficial to declare all pointers in all functions as __restrict__. To be tested in a new PR

[Luthaf]

I'm not sure whether compiler propagate restrict annotations in the backend when doing

void foo(double* __restrict__ b) {
    double* a = b;
}

If they don't, adding explicit annotations might help a bit with optimizations.