Upgrade parking_lot to 0.10 version and use MaybeUninit for it

[tyranron]

This PR upgrades parking_lot dependency to 0.10 version.

• Changelog

Additionally

Makes OnceCell to use MaybeUninit under-the-hood when parking_lot feature is enabled

Checklist

• Cargo.lock.min updated with cargo +nightly generate-lockfile -Z minimal-versions + cargo lock translate -v1.
• CI job with minimal Rust version 1.36.0 for parking_lot feature added
• rustfmted
• CHANGELOG.md entry added

[matklad]

While we are at it, it also makes sense to use MaybeUninit here:

once_cell/src/imp_pl.rs

Line 12 in ea4634c

pub(crate) value: UnsafeCell<Option<T>>,

. And, as this bumps MSRV, it should bump to 1.4.0.

[tyranron]

@matklad

While we are at it, it also makes sense to use MaybeUninit here

Maybe as a separate PR?

[matklad]

It makes sense to do this in the same PR where we bump MSRV, because that's exactly wha allows us ot use maybe uninit

[tyranron]

@matklad also, what is the proper way for you to update Cargo.lock.min to V1 lock-file format?

[matklad]

Don't know about v1, haven't looked into it yet. In general, it's best to just generate .min.lock using Cargo 1.31

[tyranron]

Hmm... I've used cargo +nightly generate-lockfile -Z minimal-versions + cargo lock translate -v1 and it seems to work well.

[tyranron]

@matklad I've landed the initial implementation via MaybeUninit for sync::OnceCell. This is majorly based on #72 (thank you, @pitdicker!).

As I have a little experience dealing with unsafes and low-level stuff like uninitialized memory, please make an early review of this. Once it's OK, I've land the similar changes for Lazy.

@pitdicker I'll be happy to see your comments too!

[tyranron]

@matklad what we have now:

• MaybeUninit is used only for parking_lot feature;
• MSRV is increased only when parking_lot feature is enabled;
• get_unchecked, get_mut and into_inner accessors implementations moved into submodules (it turned out the most reasonable way to avoid unnecessary casts and inconsistencies);
• all branch changes were splitted to separate meaningful commits.

[matklad]

bors r+

Let's land this now! I think there's a couple of tweaks I want to do, but I've also found an unrelated obscure bug which I'd love to fix, so I do the changes myself

[bors]

Build succeeded

• continuous-integration/travis-ci/push