Enumerations: how are they stored and other questions

[tczajka]

@simonbuchan

I want to emphasize here that an uninhabited type isn't quite a zero sized type; it's generally fine to treat them as such, but ideally size_of() would be returning that it has no size at all, not even a size of 0.

Ideally it should be -∞:

• 2^-∞ = 0, log 0 = -∞
• size of (u32, Never) would be 4 + (-∞) = -∞

Unfortunately -∞ doesn't fit in usize.