memorypool: add optional support for the system allocator in Rc

[jkarneges]

This enables our custom Rc type to use the system allocator instead of a memory pool. The result is a single type that works either way, which will be useful for supporting collections of elements that have mixed memory backings. For ref-counted types not in collections, or for collections of ref-counted types where all elements use the system allocator, std::rc::Rc should be preferred.

Initially I considered making this work by forwarding calls to std::rc::Rc based on a flag, but that would have required adding branching overhead to every method. Instead, this change simply adds alternative construction/destruction logic and leaves clone/deref as-is.