feature : assist delegate impl This PR ( fixes #14386 ) introduces a new IDE assist that generates a trait impl for a struct that delegates a field. This is a draft because the current `ide_db::path_transform::PathTransform` produces some unwanted results when it deals with extern crates, an example of which I attach as a GIF. GIFs : 1. A general case  2. A case where `ide_db::path_transform::PathTransform` fails to correctly resolve a property ( take `Allocator` as an example ) to its full path, thus causing an error to occur. ( Not to even mention that resolving this causes another error `use of unstable library feature 'allocator_api'` to occur 