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 1.
- We have support for dynamically-sized types (see from_header_and_iter).
- We have support for thin arcs to unsized types (see ThinArc).