Crate servo_arc

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).

Structs

• Arc
• ArcBorrow
  A “borrowed Arc”. This is a pointer to a T that is known to have been allocated within an Arc.
• HeaderSlice
  Structure to allow Arc-managing some fixed-sized data and a variably-sized slice in a single allocation.
• HeaderWithLength
  Header data with an inline length. Consumers that use HeaderWithLength as the Header type in HeaderSlice can take advantage of ThinArc.
• NonZeroPtrMut
  Wrapper type for pointers to get the non-zero optimization. When NonZero/Shared/Unique are stabilized, we should just use Shared here to get the same effect. Gankro is working on this in 1.
• RawOffsetArc
  An Arc, except it holds a pointer to the T instead of to the entire ArcInner.
• ThinArc
• UniqueArc
  An Arc that is known to be uniquely owned