Test char formatting, replace mem::uninitialized with MaybeUninit

[saethlin]

core::mem::uninitialized doesn't actually produce uninitialized memory anymore, as a mitigation. So it would be perf-equivalent or possibly faster to just use [0u8; 4] here. But just in case someone cares about the performance of this code path, I ported over some standard library internals and adapted them to this use case so we can use an uninit buffer.