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 `Send + 'static` is used as the explicit bound in most cases, where would `Own` be used? (I.e. is it useful?)