get_mut method on Outputs allows for creating multiple refererences as mutable to the same data

[PieterPenninckx]

The method get_mut on Outputs has the following signature:

pub fn get_mut(&self, i: usize) -> &'a mut [T]

Here we are creating a reference as mutable from a reference as immutable. In this way, we can get multiple references as mutable to the same data by passing the same index i more than once, violating Rust's aliasing rules.

This is similar to #66, but not fixed by my PR #67, which is why I created a separate issue.