Optimize the representation of `NotNaN`

[notgull]

Here's a thought experiment which would almost certainly be a breaking change, but something worth looking into nonetheless.

Right now, the inner representation of a NotNan<T> is a T. However, we know one thing about the value: it will never (soundly) be a NaN. Using this information, we could optimize the inner representation of the type so that Option<NotNan<T>> is a no-op. This could be done by using wrapping addition and subtraction combined with NonZeroU32/NonZeroU64. What are your thoughts on this?

[mbrubeck]

rust-lang/rfcs#3334 would be the ideal way to allow this optimization, if it is eventually accepted and stabilized.

[soooch]

fyi rust-lang/rfcs#3334 was closed in favor of rust-lang/rust#103724 which in turn was closed in favor of rust-lang/rust#107606