`parking_lot` `send_guard` feature prevents projects using the `deadlock_detection` feature downstream

[zakstucke]

Thanks for a great library!

#9 enabled the send_guard feature of parking_lot. This feature is mutually exclusive with the deadlock_detection feature:

/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/parking_lot-0.12.3/src/lib.rs:35:1
compile_error!("the `send_guard` and `deadlock_detection` features cannot be used together");

Because of this, any project wanting to use deadlock_detection, cannot use named-lock, mutually exclusive features should only be enabled by the end user, to prevent either or situations like this.

I'm stuck on 0.3 because of this at the moment, just tried to migrate to 0.4.

[oblique]

Thanks for the report. I'm trying to figure out a way of solving this without breaking compatibility.