Replace all calls to mem::uninitialized with MaybeUninit

[asomers]

Rust will deprecate mem::uninitialized beginning with Rust 1.38.0. The recommended replacement is MaybeUninit. Unfortunately, MaybeUninit was only introduced in Rust 1.36.0. So we'll have to raise the MSRV. We should therefore wait to fix this issue until shortly before 1.38.0 is released, which will be sometime around September-29.
https://blog.rust-lang.org/2019/07/04/Rust-1.36.0.html