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
So I'm not sure how to remove this submodule commit. I went into the submodule and reset it, but now I'm trying to make a new (different) pull request and that's still showing up in the changes. But what commit do I amend? I've never run into this before and I can't find any info on fixing it.