Copy safety

[kjvalencik]

Currently, it is possible to Copy and Clone Js* types which is unsafe. These types depend on being contained in a Handle to verify the lifetime. They should only exist as references outside of a Handle. Unfortunately, the ability to clone allows getting an owned copy from a reference.

This change attempts to minimize impact to users by making Handle<T> Copy, while making the inner T !Copy. This works by introducing a new marker trait TransparentWrapper which indicates a type is a transparent wrapper around a TransparentWrapper::Inner type--allowing transmutation between the two.

Using this trait, Handle actually stores a Copy inner type, while implementing Deref with a Target to the !Copy outer type.

Unfortunately, since Copy won't propagate upward from a Deref, all APIs that currently take self on a Value were changed to take &self to keep from requiring a manual clone(). This could still impact users in some places--for example when creating an Object with a circular reference.

[dherman]

This looks like a great solution. I just found the explanation confusing which made it hard to review at first. Once we improve the presentation of the trait a little bit I think it’ll be manageable from a maintenance perspective.