And the trait `OrderingOrBool<L, R>` is now for wrapped functions rather than for `Ordering` and `bool`.

NOTES:
- The new struct `MergeFuncLR<F, T>` is useful to avoid conflicting implementations of `OrderingOrBool<L, R> for F` because the compiler thinks that `F` could implement both `FnMut(&L, &R) -> Ordering` and `FnMut(&L, &R) -> bool`.
- The new trait `FuncLR<L, R>` is useful to be able to infer the Output type `T` without having to add a fourth parameter to `MergeJoinBy`. From what I understand, this is needed because the `FnMut` trait is not fully stabilized yet.
- `OrderingOrBool<L, R>` has a new associated type `Out`, which is useful in `impl Iterator` because otherwise the compiler would complain about `T` be unconstrained.

This successfully pass the tests.

Being now an alias, we can remove various implementations. `FusedIterator` will be inserted back soon.

We don't need `MergePredicate<T>` anymore because we use two new implementations of `OrderingOrBool<T, T>`.

`I` and `J` are fused by `InternalMergeJoinBy` so we don't need them to implement `FusedIterator`.

Apparently not needed anymore.