Fork of triomphe, which is a fork of 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::ArcBox 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 a (non-owned) pointer to the inner Arc. This helps avoid pointer-chasing.
* elysees::OffsetArcBorrow 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::ArcRef is a union of an Arc and an ArcBorrow