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.
`struct Xyz` is zero-sized. You can swap around instances of it as much as you want – it won't do anything.