weak reference support

This could really help me get my experimental data management project working with `no_std`: https://github.com/brodycj/sclang I don't need all of the standard functions that are supported by `std::rc::Weak`, I just need to be able to convert between strong & weak RC references as needed. It may be ideal to get this working with `Arc` but my project is not yet ready for multi-threaded support anyways (and may never be). My experimental project is working on a way to handle automatic memory management with circular data, without the random GC overhead. I make no guarantees at this point, not about correctness & not about resource efficiency, just a first experimental step. Rust's strong & weak RC reference mechanism is extremely helpful, hoping to find a way to get past this issue. From a quick look through `rc.rs` both here & in main Rust project I suspect this should not be super-difficult, and I may try hacking this myself. I just wanted to open this issue to get the discussion started. Thanks!!