Allow deriving 'Trace' on 'Copy' types

[mystor]

This change means that the Finalize::finalize method is not automatically called on drop. This might be a reasonable approach to take, and would avoid the need for a separate derive, but would be a breaking change.

Fixes #87

[Manishearth]

The diagnostics on this seem like they would be terrible :/

Plus, isn't this a behavior change? How is finalization handled after this?

[mystor]

It is a behaviour change, as I mention in the first comment, which is why I just have this marked as a draft.

And yes, the diagnostics aren't amazing.

[Manishearth]

Yeah, I'm uncomfortable about behavior changes, we do have users (like boa) and behavior changes are far nastier than compile-breaking changes.

Tbh it would be nice if we could just blanket-impl trace for copy types, but that would preclude a bunch of other things

[Razican]

The issue I have seen with doing a blanked-impl is that then the compiler prevents implementing Trace for any non-Copy type in the standard library. (so no trace for String, Vec and so on.

A #[derive(TraceCopy)] would work too, I think.

[Manishearth]

yeah, that's the stuff the blanket impl would preclude 😄