libcore can use unstable library features without a feature flag

[RalfJung]

I've been assuming that feature flags can also be used "within" the standard library to control which features are already being used. That is helpful when stabilizing a part of the API, to test if that API is viable enough for whatveer the standard library does with it.

But it turns out that, e.g., core::fmt::float can call MaybeUninit::get_mut without enabling the relevant feature flag. That seems like a bug to me.