Refactor `ResMut` and `NonSendMut` into wrappers of `Mut`

This is a follow up to the [Discord discussion](https://discord.com/channels/691052431525675048/742569353878437978/1000518580594552992) about converting `ResMut` and `NonSendMut` into `Mut`, which in itself is achieved by #5438.

During the discussion, @DJMcNab pointed that since `ResMut` and `NonSendMut` have the exact same fields and same implementation as `Mut`, and only differ by their system param role, we could have them be strict wrappers of `Mut` instead of duplicating the fields and the implementations using macros. Something like:

```rust
pub struct ResMut(Mut);
pub struct NonSendMut(Mut);
```

This incidentally makes #5438 trivial.

Start by reviewing the implementation of Mut, ResMut, and NonSendMut, along with the related change in issue #5438. Confirm how their fields and implementations are currently shared or duplicated, then verify that ResMut and NonSendMut become strict wrappers without changing their system-parameter roles or the behavior targeted by #5438.

Written by the indexing model from the issue text.