feat: add parking_lot as a feature

[quake]

resolve #137, a parking_lot feature flag is added to keep compatibility with old version.

[quake]

@xacrimon could you help to review this PR?

[pjenvey]

Why switch from --all-features to manually specifying all 4 features?

[quake]

it will report a false error regarding to feature switch:

cargo clippy --all-targets --all-features -- -D warnings

error[E0432]: unresolved import `crate::lock`
 --> src/set.rs:3:12
  |
3 | use crate::lock::RwLock;
  |            ^^^^ maybe a missing crate `lock`?

...

[dtzxporter]

I ran this on production for a while and I ran into several deadlocks, seamingly more common on rust 1.52 than 1.50. May be worth investigating...

[pjenvey]

I ran this on production for a while and I ran into several deadlocks, seamingly more common on rust 1.52 than 1.50. May be worth investigating...

@dtzxporter you're not holding Ref guards across await points, are you?

If not, were you seeing any deadlocks prior (w/out parking_lot)?

[dtzxporter]

I am not, in fact, all of the locks were held internally to my structure which didn't have any async fn's.

I've had one segfault with dashmap before, so I'm not sure if it's related, but this was happening pretty much 80% of the time for me using parking_lot.

I've ended up removing dashmap entirely and switching to parking_lot:RwLock<HashMap<>> for now and it's been fine.

[xacrimon]

Hmm, may be related to llvm. Working on a switch to parking lot entirely.

[xacrimon]

Closing this, releasing v5 with parking_lot in a hot minute.