Update to mem::MaybeUninit and Once::new()

[nipunn1313]

Newest version of rust compiler warns about deprecation for mem::uninitialized and ONCE_INIT - prefers these versions. I confirmed this compiles on the latest stable as well as the latest nightly.

[sfackler]

I'd rather not bump the minimum supported rust version for internal-only changes like this.

[nipunn1313]

Yep - I was just going to ask about that. Would it be worth throwing cfg[feature(maybe_uninit)] around it ?rust-lang/rust#63566?

[nipunn1313]

The reason this came up is because on the latest nightly - the warnings for openssl come up every time we compile our crates (even if rust-openssl didn't require a recompile) - so they tend to block up the screen.

eg

warning: use of deprecated item 'std::mem::uninitialized': use `mem::MaybeUninit` instead
   --> /home/nipunn/src/client/rust_patched_vendor/rust-openssl/openssl/src/sha.rs:285:38
    |
285 |             let mut hash: [u8; 64] = mem::uninitialized();

[nipunn1313]

see rust-lang/cargo#7450 for context

[sfackler]

Sticking #[allow(deprecated)] on the offending functions seems reasonable.

[nipunn1313]

Put up 1231 to take that approach. This can be revived eventually whenever you would like to bump the min supported rust version.