Parse const function pointer syntax

- Mentioned in https://github.com/rust-lang/rust/issues/63997:

```rust
const fn foo(f: const fn() -> i32) -> i32 {
f()
}
```

Start by reproducing the Rust snippet from the issue and determine how syn currently handles the const function-pointer syntax. Done means the example parses successfully without changing the meaning of existing function-pointer syntax; the issue does not name a file or test to begin with.

Written by the indexing model from the issue text.