Flags for unspecified behaviour

[Rendered](https://github.com/arielb1/rfcs/blob/unspecified-flags/text/0000-unspecified-flags.md)
Note that even if it wasn't mentioned in the reference, it would still be well-defined. The reference is not a language specification and does not cover most of Rust's semantics. A flaw in the reference does not change the definition of the language. The implementation is the closest thing to a specification right now.

It would be backwards incompatible to alter a guarantee that the compiler provides, even if it is not written down anywhere. Fixing undefined behaviour (not the same as unspecified behaviour) is not backwards incompatible. Any UB is an implementation soundness bug..