`MutableCloneableVector` -> `MutableCloneableSlice`

I don't know much about Rust (yet?), but I just tried to build this and got an
error about `MutableCloneableVector` not existing in `std::slice`. Looks like it
was renamed in 0.12.0 to `MutableCloneableSlice` (rust-lang/rust@4f5b692).
Merge pull request #1 from KenanY/mutable-cloneable-slice

`MutableCloneableVector` -> `MutableCloneableSlice`