Élysées

Fork of triomphe::Arc. This has the following advantages over std::sync::Arc:

• elysees::Arc doesn't support weak references: we save space by excluding the weak reference count, and we don't do extra read-modify-update operations to handle the possibility of weak references.
• elysees::UniqueArc allows one to construct a temporarily-mutable Arc which can be converted to a regular elysees::Arc later
• elysees::OffsetArc can be used transparently from C++ code and is compatible with (and can be converted to/from) elysees::Arc
• elysees::ArcBorrow is functionally similar to &elysees::Arc<T>, however in memory it's simply &T. This makes it more flexible for FFI; the source of the borrow need not be an Arc pinned on the stack (and can instead be a pointer from C++, or an OffsetArc). Additionally, this helps avoid pointer-chasing.
• elysees::Arc has can be constructed for dynamically-sized types via from_header_and_iter
• elysees::ThinArc provides thin-pointer Arcs to dynamically sized types
• elysees::ArcUnion is union of two elysees:Arcs which fits inside one word of memory

This currently just adds a getter to the refcount, but some other additional features are planned, including 3 and 4-variant ArcUnions, and ArcUnions with ThinArcs.