RFC: Memory exploit mitigation

[Rendered view](https://github.com/kmcallister/rfcs/blob/hardening/active/0000-memory-exploit-mitigation.md).
I don't really think of it as "changes to the source", since the translation from Rust source to machine code is always very complex.  I was more thinking, if there are natural things to do in codegen that would incidentally make exploits harder, we don't need to go out of our way to disable those things at `-H0`. But I'm not sure what would go in that category, and I do see the appeal of a setting for "make compilation as simple as possible", even if it's still not very simple :)