Flags for unspecified behaviour

[Rendered](https://github.com/arielb1/rfcs/blob/unspecified-flags/text/0000-unspecified-flags.md)
There's a difference between *undefined behaviour* and something that's left unspecified. It is explicitly specified as being undefined behaviour. That means a program doing it is memory unsafe / incorrect and the language does not provide any guarantees about what will happen.