Use HRTBs for function traits

[udoprog]

This is necessary to avoid unsound use of references in registered functions. An issue that was discovered when investigating #600.

The way function arguments are currently modeled uses unbound references, which among other things allow you to register a 'static reference or any associated lifetime allowing values to escape:

#[rune::function]
fn download(url: &'static str) {
    todo!()
}

struct Foo<'a> {
    url: &'a str,
}

impl<'a> Foo<'a> {
    fn download(url: &'a str) {
        self.url = url;
    }
}

let module = Module::new();
module.function_meta(download);

With this change, functions like the above are correctly rejected instead. A downside is that we can no longer use references for arguments in async functions. Instead these must use owned types:

#[rune::function]
async fn download(url: &str) {
}

Becomes:

#[rune::function]
async fn download(url: Ref<str>) {
}

And associated types like this:

struct Foo;

impl Foo {
    #[rune::function]
    async fn download(&self, url: &str) {
    }
}

Becomes:

struct Foo;

impl Foo {
    #[rune::function(instance, path = Self::download)]
    async fn download(this: Ref<Self>, url: Ref<str>) {
    }
}