Flags for unspecified behaviour

[Rendered](https://github.com/arielb1/rfcs/blob/unspecified-flags/text/0000-unspecified-flags.md)
This is actually well-specified as wrapping around: http://doc.rust-lang.org/reference.html#behaviour-not-considered-unsafe
I suppose this prevents `x + 1 > x` and `(2*x) / 2` from being optimized to `true` and `x` respectively, and it might be interesting to investigate if such optimizations would really make existing Rust code faster. But for now it is well-specified.