RFC: Improve the Send trait.

[Rendered view](https://github.com/pythonesque/rfcs/blob/send-improvements/text/0000-send-improvements.md).

This RFC proposes extending the ```Send``` trait in some relatively small but backwards incompatible ways, to allow more safe behaviors to be exposed in Rust's type system.  In particular, this involves removing the ```'static``` bound from Send in a way that preserves thread safety.
If `Own` is just defined literally as a synonym for `Send + '`static`, i.e.

    trait Own: Send + 'static { }
    impl<T: Send + 'static> Own for T { }

then I don't think either issue arises. (This is basically the definition from the RFC, but I'm not sure why we'd need `unsafe`?)