subscriber: rm crossbeam-utils, make parking_lot opt-in

[hawkw]

Motivation

The tracing-subscriber currently depends on both the crossbeam-util
crate (for its ShardedLock type) and the parking_lot crate for it's
RwLock implementation. The Rust standard library also provides a
RwLock implementation, so these external dependencies are not strictly
necessary. In the past, using these crates was anecdotally observed to
result in a performance improvement, but the tradeoff is adding
additonal crates (and their transitive dependencies) to the user's
dependency tree.

Solution

This branch removes the crossbeam-util dependency, and makes the
parking_lot dependency opt-in. The explicit use of
parking_lot::RwLock with a wrapper type that abstracts over the
parking_lot::RwLock and std::sync::RwLock types to provide a
matching API. This allows the parking_lot feature flag to
transparently replace the use of std::sync::RwLock with parking_lot,
rather than making it required for other features.

[hawkw]

Switching from crossbeam-util::ShardedLock to std::sync::RwLock in EnvFilter doesn't seem to have a noticeable impact on the quick benchmarks I added in ade986b --- the dynamic/multithreaded benchmark (which actually exercises locking in the filter) actually shows a slight performance improvement on this branch vs master (from 15us to 12us), though this may not be significant.