rename MutableVector::mut_split(at) to MutableVector::mut_split_at(at)
add MutableVector::mut_split(self, pred) -> DoubleEndedIterator<&mut [T]>

This method is the mutable version of ImmutableVector::split.  It is
a DoubleEndedIterator, making mut_rsplit irrelevent.  The size_hint
method is not optimal because of #9629.

At the same time, clarify *split* iterator doc.
auto merge of #10757 : TeXitoi/rust/mut-split-iter, r=alexcrichton

I've renamed `MutableVector::mut_split(at)` to `MutableVector::mut_split_at(at)` to be coherent with ImmutableVector.  As specified in the commit log, The `size_hint` method is not optimal because of #9629.