Make TryLock::new const

[Chris--B]

I was playing with allocators and wanted a lightweight lock to protect some state. I found this crate, works great. Only complaint is that to initialize a GlobalAllocator, I needed ::new() to be const. I made a small change here and it works!

TryLock::new calls two other functions, but is otherwise already const-able.

• AtomicBool::new, const since 1.24
• UnsafeCell::new, const since 1.32

Since both functions have been const for a long time, I would be surprised if this change breaks anyone's code. Most projects track the latest stable Rust, so in my opinion this is fine.

I also tried consting other functions, but it does not work on stable. The big blocker are the panic!() calls in TryLock::try_lock_explicit_unchecked. I didn't need anything else to be const, so I left everything else alone.

[Chris--B]

Well, I see 1.21 in the PR checks, so I can already tell this will fail that build. This change bumps the minimum needed to 1.32, from January 2019.

[Chris--B]

Oh, this is embarrassing. I thought I checked but completely missed PR #2! They hit on the same issues and even update the tests & code.

[Chris--B]

I also tried consting other functions, but it does not work on stable. The big blocker are the panic!() calls in TryLock::try_lock_explicit_unchecked.

This has landed in stable, without formatting. So more of these might be const-able now or soon.

[Finomnis]

If this PR is a duplicate, then it might be a good idea to close it.

[seanmonstar]

Done in #2