Feature request: emit `Fn` impls for functions

Functions and function pointers can be used where closures are expected:
```rust
self.partial_cmp(other).is_some_and(Ordering::is_gt)
```
he way this works is that every function item is assigned a unique (unnameable) type, and that type implements the `Fn` traits. This impl is a builtin impl, today, it would be nice to emit a real impl instead.