feat: add impl-level exporting QoL macro

[HoloTheDrunk]

This adds a new macro that allows for more concise exporting of a lot of functions. Currently in a sort of PoC state, a lot can and will be improved.

Why ?

Having to edit multiple places when making simple changes to an API is tedious and a source of error.

How ?

#[derive(rune::Any)]
struct MyStruct(#[rune(get)] usize);

// Creates a function that returns an array of all the function metadata
#[rune::item_impl]
impl MyStruct {
    #[rune(export)]
    pub fn succ(&self) -> Self {
        Self(self.0 + 1)
    }
}

Roadmap

• [x] Proper documentation
• [x] Non-instance function handling

I had to add the extra-traits syn feature, if you know of a way to do the comparison on crates/rune-macros/src/item_impl.rs:70 without it that'd be pretty cool.

[HoloTheDrunk]

If I haven't missed anything, the PR should be ready for review whenever you've got time :) edit: oops, I had in fact forgotten things

[HoloTheDrunk]

Hi! Thank you for the review and sorry for the delay.

The main motivation for grouping things like this is to avoid the need to have a human error-prone process when altering a type's API as you jump between definition and export locations. Most projects will likely have pretty simple export configurations so this method also streamlines the experience when trying out the library for example, leaving the specifics for later (e.g. I originally expected keep to be the default behaviour for function exports and almost dropped the idea of using Rune when it wasn't mentioned anywhere simply due to the added maintenance time cost).

Though using a trait would be much cleaner, it'd make it impossible to export multiple impl blocks using this method; I don't know how much of a problem that is as I usually don't split my impls but I don't have any data on how common/useful that practice is to others. If it's not considered a concern for the scope of Rune I'll gladly replace everything with a trait and a corresponding module function.

I'll make those changes within the a couple of days if/when I get your go on this :)