Make TryLock::new const fn

[yvt]

Enables the use of TryLock::new in a constant context.

Requires Rust 1.31.0 or later and discontinues the support for the older versions. (I'd like to hear your opinion on this - Do you still intend to support Rust 1.21? Or can we drop the support after a major version bump?)

[chrysn]

@seanmonstar, could you have a look at this? Apart from the constness, it looks like a TryLock would be very useful for static items.

[Chris--B]

Pinging here, because I ran into the exact issue and would love to use a crates.io version of this instead of a GitHub fork. Using this in static items effectively requires TryLock::new be const, and moving the MSRV up from very old to pretty old seems like an easy ask imo.

[Finomnis]

@seanmonstar Any thoughts on this? Two years later now, bumping the min version to 1.31 really doesn't seem like much of an issue any more :)
I'm also in favor of removing the deprecated function and releasing a 1.0 version, since this crate seems quite battle tested. (>90 Mil downloads on crates.io)

[seanmonstar]

1.21 to 1.31 is a pretty reasonable change at this point. It's still several years ago.