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.
`Own` wasn't part of the original proposal.  t added it after Niko suggested it might be useful for, e.g., where `Box<Send>` is used today, but as I commented earlier today I'm not sure how useful this really is.  I don't really see any harm in its existence either way.