[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:
That sounds like a plan. However, our `::memset` doesn't seem to ever return a negative value. Here I am really expecting `-13`, but this gives me `243`, which is indeed a `-13 as u8`...

I think this is the fix:
```diff
-            return (a - b) as i32
+            return (a as i32 - b as i32)
```