Getter Macro Improvements

[soareschen]

Summary

This PR brings improvements to the #[cgp_getter] macro to support new kinds of getter fields to be derived.

Slice Field

A getter method that returns slice reference, e.g. &[u8] can now be implemented with field types that implement AsRef.

Example:

#[cgp_auto_getter]
pub trait HasBytes {
    fn bytes(&self) -> &[u8];
}

Maybe Reference

A new MRef type is introduced to allow either reference or owned value to be returned:

pub enum MRef<'a, T> {
    Ref(&'a T),
    Owned(T),
}

This type will be useful for custom field getter implementations, so that they can return owned values in case when it is a virtual field that is not directly present within a context.

The default UseField implementation will still require an actual field to be present, and use MRef::Ref to return the borrowed value.

Example:

#[cgp_getter {
    provider: NameGetter,
}]
pub trait HasName {
    fn name(&self) -> MRef<'_, String>;
}