Merge #4493

4493: Provide builtin impls of Fn traits for fn-pointers r=flodiebold a=hban

Meant to be, but isn't actually a fix for #2880.

Consider this snippet:

```rust
use std::marker::PhantomData;
use std::ops::Deref;

struct Lazy<T, F/* = fn() -> T*/>(F, PhantomData<T>);

impl<T, F> Lazy<T, F> {
    pub fn new(f: F) -> Lazy<T, F> {
        Lazy(f, PhantomData)
    }
}

impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
    type Target = T;

    fn deref(&self) -> &T { todo!() }
}

fn test() {
    let lazy1: Lazy<u32, _> = Lazy::new(|| 0u32);
    let r1 = lazy1.to_string();

    fn make_u32_fn() -> u32 { todo!() }
    let make_u32_fn_ptr: fn() -> u32 = make_u32_fn;
    let lazy2: Lazy<u32, _> = Lazy::new(make_u32_fn_ptr);
    let r2 = lazy2.to_string();
}
```

* On current master:
  * When type default is commented-out, `r1` is correctly inferred, `r2` in _{unknown}_.
  * When type default is not commented-out, both `r1`  and `r2` are _{unknown}_.
* With this PR:
  * When type default is commented-out, both `r1` and `r2` are correctly inferred.
  * When type default is not commented-out, both `r1`  and `r2` are _{unknown}_.

Well, it's a improvement at least. I guess this thing with type defaults is a different problem.

I also tried add Fn impls for fn items, but wasn't successful. So this PR only adds those impls for fn pointers.

Co-authored-by: Hrvoje Ban <hban@users.noreply.github.com>

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