Introduce #[rune::function] to capture meta from native fns

[udoprog]

With this change, #[rune::function] can be used to annotate native functions, which will allow rune contexts to capture their metadata such as documentation strings and name of arguments.

The intention is to capture this in the Context so it can be used to generate documentation. But as a side effect, #[rune::function] can statically determine the kind of function to register so this could potentially reduce the number of methods needed for Module in future versions.

use rune::{Any, Module, ContextError};

/// This is a pretty neat function.
#[rune::function]
fn function(string: &str) -> String {
     string.to_string()
}

/// This is a pretty neat async function.
#[rune::function]
async fn async_function(string: &str) -> String {
     string.to_string()
}

#[derive(Any)]
struct Test;

impl Test {
    /// This is a pretty neat async instance function.
    #[rune::function]
    fn function(&self, string: &str) -> String {
         string.to_string()
    }

    /// This is a pretty neat async instance function.
    #[rune::function]
    async fn async_function(&self, string: &str) -> String {
         string.to_string()
    }
}

fn module() -> Result<Module, ContextError> {
     let mut m = Module::new();
     m.function2(function)?;
     m.function2(async_function)?;
     m.ty::<Test>()?;
     m.function2(Test::function)?;
     m.function2(Test::async_function)?;
     Ok(m)
}