Indexing a C array #609

We need to be able to generate the following code in the C backend:

void f(uint16_t *q) {                                                  
    for (size_t i = 0; i < 10; i++) {
        g(&q[i])
    }
}

How should the Python code look like?

One idea:

@ccall
def g(a: CPtr) -> None:
    pass

@ccallable
def f(q_void: CPtr) -> None:
    q: Pointer[i16[:]]
    c_p_pointer(q_void, q)
    for i in range(10):
        q2: CPtr
        p_c_pointer(pointer(q[i]), q2)
        g(q2)

In LPython we can support this, but in CPython it might not be possible, so maybe we need to change pointer(q[i]) to pointer_index(q, i) or something like that.