Support optional arguments

[ghost]

As @karroffel mentioned that a new cargo version is coming soon, I think it might be a good time to move this out of draft status, so the feature might get into the release!

Implements optional arguments in both godot_wrap_method and the methods procedural macro. Closes #185.

Explanation

This patch allows declaring optional arguments with the following syntax:

With godot_wrap_method:

builder.add_method("some_method", godot_wrap_method!(
    SomeType,
    fn some_method(&self, owner: Node, foo: i64, #[opt] bar: Option<String>, #[opt] baz: Variant) -> ()
));

With the procedural macro:

#[methods]
impl SomeType {
    #[export]
    fn some_method(&self, owner: Node, foo: i64, #[opt] bar: Option<String>, #[opt] baz: Variant) -> () {
        let bar = bar.unwrap_or_else(|| "DefaultValue".to_owned());
        // ... do things with bar ...
    }
}

Default values for optional arguments are obtained with Default. The behavior regarding Option<T> is consistent with its FromVariant implementation, so it should be easy to convert an existing function into one with optional arguments.

Future possibilities

VarArgs methods

With attribute argument parsing in place, it should be easy to later extend the syntax to allow VarArgs methods. Sample:

#[methods]
impl SomeType {
    /// foo is required, bar and baz are optional, and rest contains the rest of arguments if any remains
    #[export]
    fn some_method(&self, owner: Node, foo: i64, #[opt] bar: Option<String>, #[opt] baz: Variant, #[var] rest: VariantArray) -> () {
        // - snip -
    }
}

[ghost]

Added procedural macro implementation and ready for review now!

[karroffel]

This looks fine all in all, I am not too convinced about the attribute. I would say we wait until after the next release and give this some more testing?

[ghost]

Ok. That's reasonable.

[ghost]

Rebased on #274, will probably switch to the alternative syntax now that parameter attributes are stable for a few versions. Using a shorter attribute like #[opt] might help reduce the noise.

[ghost]

Updated the PR to use the argument attribute syntax, with the attribute #[opt].

@karroffel Could you review again?

[karroffel]

Looks good to me. Nice work!