Add a test for taking a CPtr pointer of a struct (dataclass) from Python

[certik]

Using the p_c_pointer approach.

[certik]

Currently the only way seems to be to allocate the memory in C and then use as follows:

        sptr: CPtr = my_allocate(sizeof(S))
        s: Pointer[S] = c_p_pointer(sptr, S)
        s.k = 5

But we would like something like:

s: S = S()
s.k = 5
sptr: CPtr = p_c_pointer(s)

[czgdp1807]

Note the differences.

sptr: CPtr = my_allocate(sizeof(S))
        s: Pointer[S] = c_p_pointer(sptr, S)
        s.k = 5

This would do a call to C function my_allocate which further calls malloc and then returns a pointer which gets assigned to sptr.

s: S = S()
s.k = 5
sptr: CPtr = p_c_pointer(s)

Here S gets created using alloca LLVM instruction then it is bit casted to void* and finally assigned to sptr.

@certik I think the first one already works. If you think the second one that is creating struct using LLVM's alloca is correct behaviour then I will go ahead with that.