* core: implemented `copy` protocol; most basic plain types are copyable by default; types that implement `__copy` can be duplicated using the copy protocol.
* `Rc`: retired `Weak.clone` and `Rc.clone` in favor of copy protocol
* `tupleof` constructs a viewed tuple when some arguments are views
* `Capture`: proper handling of all-unique and some-view capture arguments
* `Capture`: ensure captured values are dropped when capture is dropped