Nim's YRC: A safe multi-threaded cycle collector

I’m very happy to see this development, it addresses a pain point with Nim that existed before and after the release of ARC/ORC (Nim v2, more or less).

I don’t want to even hazard a guess how many times folks new to Nim would ask “How do I work with ref across threads without running into SIGSEGV?”, and the answer might as well have been to become a Jedi master unless the use case fit very well with one/more (let’s be honest, experimental) structured concurrency libraries. Sometimes that would turn out to be the case, or could be the case if they would pretzel their mental model for the business logic, but generally it was a big turnoff.