[RFC] Adding 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.

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 :+1:
I would prefer this test to be something like:

```rust
fn memset_strings() {
     let mut buffer = [b'X', .. 100];
     ::memset(buffer.as_mut_ptr(), b'#', buffer.len());
     for byte in buffer.iter() { assert_eq!(*byte, b'#') }
}
```

(You'll need to import the `std::slice::MutableVector` and `std::slice::ImmutableVector` traits.)