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.
This is more or less an information dump of an IRC session between @nikomatsakis , arielb1, and myself--I added it to the RFC because Niko said he wanted to make sure he remembered what we were talking about when scanning it.  I can clarify further if you want, but essentially there are various safe types that make it hard to make a more intuitive version of `Send` and `Sync` than what we currently have.