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

• triomphe::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.
• triomphe::UniqueArc allows one to construct a temporarily-mutable Arc which can be converted to a regular triomphe::Arc later
• triomphe::OffsetArc can be used transparently from C++ code and is compatible with (and can be converted to/from) triomphe::Arc
• triomphe::ArcBorrow is functionally similar to &triomphe::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.
• triomphe::Arc can be constructed for dynamically-sized types via from_header_and_iter
• triomphe::ThinArc provides thin-pointer Arcs to dynamically sized types
• triomphe::ArcUnion is union of two triomphe:Arcs which fits inside one word of memory

This crate is a version of servo_arc meant for general community use.