#[carol] should generate a remote calling interface

[LLFourn]

Take this test for example:

#[carol]
impl Foo {
    #[activate]
    pub fn add(&self, lhs: u32, rhs: u32) -> u32 {
        lhs + rhs
    }

    #[activate]
    pub fn checked_sub(&self, lhs: u32, rhs: u32) -> Option<u32> {
        lhs.checked_sub(rhs)
    }

    pub fn non_activate(&self) {
        unreachable!()
    }

}

use carol_activate::{Activate, Add, CheckedSub};


#[test]
fn call_add() {
    let method = Activate::Add(Add { lhs: 7, rhs: 3 });
    let call = bincode::encode_to_vec(method, bincode::config::standard()).unwrap();
    let foo = Foo;
    let output = Foo::activate(
        bincode::encode_to_vec(foo, bincode::config::standard()).unwrap(),
        call,
    );
    let (result, _): (u32, _) =
        bincode::decode_from_slice(&output, bincode::config::standard()).unwrap();
    assert_eq!(result, 10);
}

In that test I think I should be able to just do:

    let foo = Foo;
    let (input, decoder) = input_gen::add(7, 3);
    let output = Foo::activate(
        bincode::encode_to_vec(foo, bincode::config::standard()).unwrap(),
        call,
    );
    let output = decoder.decode(output)?;
    assert_eq!(output, 10);

where input_gen is codegen'd by the macro.

This is useful so you can easily activate from client code without having to construct enum variants manually.