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 said it couldn't be constructed in safe code, which is true (and we don't have any standard library types that work like this).  But it might still be possible to write safe types that use `&'static mut`.  This type is thanksto arielb1, so please ask him.