Add get_mut methods to collection types

[TomAFrench]

I noticed in AztecProtocol/aztec-packages#10556 that when wanting to mutate a nested value in a BoundedVec it's necessary to use a pattern like the below:

let mut nullifier = builder.previous_kernel.nullifiers.get(2);
nullifier.nullifier.note_hash = 9988;
builder.previous_kernel.nullifiers.set(2, nullifier);

Ideally we'd support mutating the returned element to allow something like the below by returning a mutable reference to the element.

builder.previous_kernel.nullifiers.get_mut(2).nullifier.note_hash = 9988;

[jfecher]

This'd require some large SSA changes since we don't support getting a mutable reference to an array element at all currently. Only ArrayGet which conceptually returns a copy