|| Fork of Arc for Servo. This has the following advantages over std::sync::Arc:
* We don't waste storage on the weak reference count.
* We don't do extra RMU operations to handle the possibility of weak references.
* We can experiment with arena allocation (todo).
* We can add methods to support our custom use cases .
* We have support for dynamically-sized types (see from_header_and_iter).
* We have support for thin arcs to unsized types (see ThinArc).
* We have support for references to static data, which don't do any