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.
I think you're right and we don't need unsafe.  That definition went through several iterations.  I'll adjust the RFC.  However, I still don't think we want to use `Own` as an explicit bound, because it is still possible that someone could deliberately write a negative impl for `Own` (I'm not sure *why* you would though, so maybe that's not really a case worth considering).