Fix data race possible from safe code on InlineArray type

[Ollie-Pearce]

The make_mut function for InlineArray selects the wrong From implementation as calling self.deref() matches the blanket impl<T> Deref for &mut T first before reaching InlineArray's own Deref impl.

impl From<&InlineArray> for InlineArray { //Incorrect From impl
    fn from(v: &Self) -> Self { v.clone() }
}

impl From<&[u8]> for InlineArray { //Correct From impl
    fn from(slice: &[u8]) -> Self { InlineArray::new(slice) }
}

Reproduction:

Test case:

#[test]
fn make_mut_races_a_shared_clone() {
    let bytes: InlineArray = InlineArray::from("foobar000".to_string());
    let bytes_clone = bytes.clone();

    std::thread::scope(|s| {
        s.spawn(move || {
            let bytes = bytes;
            bytes.as_ref();
        });

        let mut bytes = bytes_clone;
        bytes.make_mut();
    });
}

Verifying with Miri:

cargo +nightly-2025-08-20 miri test --test repro

test make_mut_races_a_shared_clone ... error: Undefined Behavior: Data race detected between (1) retag write on thread `make_mut_races_` and (2) retag read of type `[u8]` on thread `unnamed-2` at alloc40180
    |
276 |                 std::slice::from_raw_parts(data_ptr, len)
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ (2) just happened here
    |
help: and (1) occurred earlier here
   --> tests/repro.rs:15:9
    |
 15 |         bytes.make_mut();
    |         ^^^^^^^^^^^^^^^^
    = help: retags occur on all (re)borrows and as well as when references are copied or moved
    = help: retags permit optimizations that insert speculative reads or writes
    = help: therefore from the perspective of data races, a retag has the same implications as a read or write
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: BACKTRACE (of the first span) on thread `unnamed-2`:

Fix:

• Change self.deref() to &**self in order to invoke InlineArray's own Deref impl and the correct From impl