Make refcount version of the rrb-tree

[hypirion]

Providing a refcount variant will make portability to non-GC languages (Rust, C++, etc.) easier.

[difranco]

How about hazard pointers as in this hamt implementation? (Not a rhetorical question as I do not know myself if it would be appropriate.) https://github.com/ballard26/concurrent-hamt

[hypirion]

That's a valid question: It could be interesting for a concurrent RRB-tree, but this version is (currently) only immutable and transient, so it would incur more overhead than basic refcounting.

It sounds like something worth looking into in the future though!

[difranco]

It is a tempting thought because together with the ctrie it could be used to create a Lua table like universal map data structure with lock-free concurrency, then maybe build Iacono's working set structure on top to get cache-obliviousness.