Add unit test for rlibc

We are pulling in rlibc into Zinc (hackndev/zinc#113), but while reading the code, I though it could do with some unit tests.

<strike>So I have added some tests here, but seem like there are a few issues... Please see comments with `XXX` and respond if you have any thoughts...</strike>
"strings". I also think this test would be clearer just using arrays. There's not really any reason to use strings, and they just require contortions with `CString`.

I.e.

```rust
#[test]
fn memcpy_and_memcmp_array() {
    let src = [b'X', .. 100];
    let dst = [b'Y', .. 100];

    assert!(::memcmp(src.as_ptr(), dst.as_ptr(), src.len()) != 0);
    ::memcpy(dst.as_mut_ptr(), src.as_ptr(), src.len());
    assert_eq!(::memcmp(src.as_ptr(), dst.as_ptr(), src.len()), 0);
}
```