Support C structs

[certik]

We want to be able to represent an equivalent of the following code in Python and generate the same C code using the C backend:

struct A {
    int32_t x;
    float y;
};

void f(struct A *a) {
    int32_t x = a->x;
    float y = a->y;
    ...
}

Here is one approach how the Python code could look like:

@dataclass
class A:
    x: i32
    y: f32

@ccallable
def f(a: CPtr) -> None:
    x: i32
    y: f32
    a2: Pointer[A]
    c_p_pointer(a, a2)
    x = a2.x
    y = a2.y

It looks like two prerequisites are needed before we can tackle this:

• Implement structs using dataclasses (Add structs using dataclasses #607)
• Implement Pointer to structs (Pointer to structs #608)

[certik]

We can have pointer to structs at the C level. I keep the issue #608 open for a general solution using Pointer. Not a high priority right now.