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>
`memset_array` (The type `[X, .. n]` is an array, `&[T]` is a slice, `Vec<T>` is a vector; in this case, `buffer` is an array (although calling it a slice would be fine too).)