Lib.rs

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).
• We have support for references to static data, which don't do any refcounting.