Suggestion for garbage collection library

[sebffischer]

I stumbled upon dumpster which implements a GC that supports multi-threading.
Because we eventually want multi-threading this library might be a good candidate to use.
There is also rust-gc but there is only experimental multi-threading support in a PR.

In general, adding such a GC should not be too difficult as I believe it is mostly switching out Rc and RefCell for their gc-ed counterparts.