Remove uninitialized() in favor of MaybeUninit?

[icefoxen]

Basically, there's some points where uninitialized!() gets used when we are implementing conversions that could be turned into MaybeUninit. This looks pretty straightforward. The real question is, since this only got stabilized in rustc 1.36, is it desirable to require that as a minimum version for Mint? Not sure if that counts as a semver breaking change.