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
Hey Alex,
I made the changes you requested. Hopefully I've correctly interpreted your comment about adding my docs for next() to the trait. I took this to mean, showing both examples in the documentation for permutations in the trait definition.