Represent equivalent `enum`s the same as `bool`, make `bool` an `enum`

[CLICKME](https://github.com/glaebhoerl/rfcs/blob/make-bool-an-enum/active/0000-make-bool-an-enum.md)
I [tried to address](https://github.com/rust-lang/rfcs/pull/330/files#diff-7af99f8ead66a6b09b087392f1560216R45) this under "Arguments against". Do you have a counter-counterargument? I know we *do* currently reserve this right, but I don't see how the flexibility is at all useful in this case.