New assist: Generate `IndexMut` implementation from `Index` implementation

Given an existing core::ops::Index implementation, this assist would generate a copy of it where

• Index becomes IndexMut
• The method signature is updated
• & in the return value position in the function body (and perhaps other obviously-relevant positions) is replaced with &mut

For an example from the code I'm working on right now,

pub enum Axis { X = 0, Y = 1, Z = 2 }

impl<T> ops::Index<Axis> for [T; 3] {
    type Output = T;

    fn index(&self, index: Axis) -> &Self::Output {
        &self[index as usize]
    }
}

could be used to generate

impl<T> ops::IndexMut<Axis> for [T; 3] {
    fn index_mut(&mut self, index: Axis) -> &mut Self::Output {
        &mut self[index as usize]
    }
}

with no manual changes instead of 6. There might be other ref/mut trait pairs that could be assisted similarly, too.