remove ref-counting Gc<T>

### Summary

Remove the reference-counting based `Gc<T>` type from the standard
library and its associated support infrastructure from `rustc`.

Doing so lays a cleaner foundation upon which to prototype a proper
tracing GC, and will avoid people getting incorrect impressions of
Rust based on the current reference-counting implementation.

[(rendered)](https://github.com/pnkfelix/rfcs/blob/fsk-remove-refcounting-gc-of-t/active/0000-remove-refcounting-gc-of-t.md)