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'm not actually sure whether it's possible or not--I might have meant to switch this (that is, it is possible that a user could have `Send` implemented, but not `Own`, not the other way around, in which case we'd still want to allow it to be sent across threads).