mem::uninitialized should be replaced with MaybeUninit

[PatchMixolydic]

mem::uninitialized is deprecated, as it is known to cause immediate undefined behaviour. To allow users to work with uninitialized memory in a memory-safe manner, MaybeUninit (tracking issue rust-lang/rust#53491) was stabilized in Rust 1.36. A previous attempt to update this crate to use MaybeUninit, #1230, was rejected since it would change the MSRV for an internal-only change. However, the MSRV was bumped to 1.36 in #1400, which means that this change can now be landed.