Suggestion for garbage collection library

I stumbled upon [dumpster](https://github.com/claytonwramsey/dumpster) which implements a GC that supports multi-threading.
Because we eventually want [multi-threading](https://github.com/dgkf/R/issues/81) this library might be a good candidate to use.
There is also [rust-gc](https://github.com/Manishearth/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.

Start by reviewing the dumpster and rust-gc projects linked in the issue, then read the linked multi-threading issue for the broader requirement. Search the repository for Rc and RefCell usage to assess the integration scope. Done would require a decided GC approach and an agreed implementation plan.

Written by the indexing model from the issue text.