Implement `Default` for `fn` types and closures.

I could avoid some unsafe code if the types of functions and closures-without-captures implemented the Default trait.

Theoretically you could extend this to closures that only capture things implementing Default, but starting with only closures that have no captures seems like a safer bet, and it can always be extended later.

Example:

fn call_by_type<F: Fn() + Copy + Default>() {
    let f = F::default();
    f();
}

fn wrapper<F: Fn() + Copy + Default>(_f: F) {
    call_by_type::<F>();
}

fn foo() {
    println!("Hello, world!");
}

fn main() {
    wrapper(foo);
}

Start with the issue's closure and function-pointer example, then review the Rust RFC process in this repository to determine the required proposal and design work. The scope currently covers captureless closures and function types, with captured closures left as a possible later extension; done means a settled design and an accepted or rejected RFC outcome.