Fork of Arc for Servo. This has the following advantages over std::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).