To get rid of `const SAME: bool`, I wrap the function `F` in `MergeFuncLR` and `MergeFuncT`.
Those `MergeFuncLR/T` have a parameter  `T` (phantom field) to not have conflicting implementations of `MergePredicate<L, R>`.

In `merge_join_by` definition, in order for the compiler to keep guessing left/right types, I kept `F: FnMut(&Self::Item, &J::Item) -> T`. But to not add `F: MergePredicate<Self::Item, J::Item>` that felt duplicate, I removed `T: OrderingOrBool...`, the user might lose `OrderingOrBool` information.

And I simplify `MergeFuncLR` and `MergeFuncT` to "tuple structs".

Basically because `PutBack<Fuse<I>>` is fused no matter if `I` is or not. Same for `J`.