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 think I've now managed to parse this correctly, but perhaps it could be reworded for greater clarity. In particular I was having trouble with "but a mutable reference could not" - could not what? (Be thread-safely shared.)

And "It could be thread-safely shared provided that a `&mut` reference was not sent to another thread" - but what if one was? (The answer is that it couldn't have been, because `&mut T` is not `Send`. Again, this was stated, but it took me a while to understand.)