Add support for field functions and deriving default implementations

[udoprog]

This introduces "field functions", which are functions that can be registered to operate over field accesses. The most famous of which are getters and setters.

The Any derive can also generate default implementations of these field functions through various #[rune(...)] attributes:

#[derive(Any, Debug, Default)]
struct Foo {
    #[rune(get, set, copy)]
    number: i64,
    #[rune(get, set)]
    string: String,
}

Once registered, this allows Foo to be used like this in Rune:

pub fn main(foo) {
    foo.number = foo.number + 1;
    foo.string = `${foo.string} World`;
}

The full list of available field functions and their corresponding derives are:

• #[rune(get)] - for field getters (Protocol::GET).
• #[rune(set)] - for field setters (Protocol::SET).
• #[rune(add_assign)] - the += operation (Protocol::ADD_ASSIGN).
• #[rune(sub_assign)] - the -= operation (Protocol::SUB_ASSIGN).
• #[rune(mul_assign)] - the *= operation (Protocol::MUL_ASSIGN).
• #[rune(div_assign)] - the /= operation (Protocol::DIV_ASSIGN).
• #[rune(bit_and_assign)] - the &= operation (Protocol::BIT_AND_ASSIGN).
• #[rune(bit_or_assign)] - the |= operation (Protocol::BIT_OR_ASSIGN).
• #[rune(bit_xor_assign)] - the ^= operation (Protocol::BIT_XOR_ASSIGN).
• #[rune(shl_assign)] - the <<= operation (Protocol::SHL_ASSIGN).
• #[rune(shr_assign)] - the >>= operation (Protocol::SHR_ASSIGN).

The manual way to register these functions is to use the new Module::field_fn function. This clearly showcases that there's no relationship between the field used and the function registered:

struct External {
}

impl External {
    fn field_get(&self) -> String {
        String::from("Hello World")
    }
}

let mut module = Module::empty();
module.field_fn(Protocol::GET, "field", External::field_get)?;

Would allow for this in Rune:

pub fn main(external) {
    println!("{}", external.field);
}

TODO

Make it possible to overload the implementation function by specifying an argument to the given operation, like: #[rune(add_assign = "my_add_assign")].