API docs/examples for std::slice

 - API example for move_from in MutableVectorAllocating
 - API doc/example for next() in Permutations
 - API doc/example for permutations() in ImmutableCloneableVector
Documentation on trait method implementation shows up in rustdoc, but it's not always readily visible, and isn't necessarily the first place that someone would look for the documentation. This is a good example, though, so could you move it to the `permutations` method itself?

Also, could you use `vec![...]` instead of `vec!(...)`? (our current style)