feat(engine): add feature for trait item defaults

[paulmure]

Address #624 in the importer.

This PR adds a TIDefault variant to trait_item' in the AST to support trait items with default values/implementations.

Currently, only defaults in const and fn are supported where consts are encoded with a arity-0 function just like normal trait items.

Defaults for associated types are not currently supported as they are still an unstable feature in upstream rust rust-lang/rust#29661.

This addition is feature gated with the trait_item_default feature and all backends have been adjusted to reject this feature for the moment.

[W95Psp]

Thank you! That looks good to me, and that's a good thing to add support for, thanks! 😃