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 also just reread the comment and I actually got what I meant backwards--`Own` can't be implemented unless `Send` is.  I meant to refer to the other case (a user implementing `Send` but forgetting to implement `Own`).