Fix #3499.

This is the other half of #5841 that was left out when I revised it to #6214.

…r=camsteffen

Restriction lint for function pointer casts

The existing lints for function pointer casts cover the cases where a cast is non-portable or would result in truncation, however there's currently no way to forbid numeric function pointer casts entirely.

I've added a new lint `fn_to_numeric_cast_any`, which allows one to ban _all_ numeric function pointer casts, including to `usize`. This is useful if you're writing high-level Rust and want to opt-out of potentially surprising behaviour, avoiding silent bugs from forgotten parentheses, e.g.

```rust
fn foo() -> u32 {
    10
}

fn main() {
    let _ = foo as usize; // oops, forgot to call foo and got a random address instead!
}
```

~~I'm open to suggestions for the naming of the lint, because `fn_to_numeric_cast_any` is a bit clunky. Ideally I'd call this lint `fn_to_numeric_cast`, but that name is already taken for the more specific lint~~. We've stuck with `fn_to_numeric_cast_any` to avoid renaming the existing lint, or choosing a different name that's too generic (like `fn_cast`).

I'm also open to changing the suggestion behaviour, as adding parentheses is only one of many possible ways to fix the lint.

changelog: add ``[`fn_to_numeric_cast_any`]`` restriction lint