Fix new nightly Clippy warnings

[notgull]

• Suppress warning from the cfg(loom) directive by having it emit "rustc-check-cfg" in build.rs
• Remove usage of legacy "std::usize" module.
• Ignore "clippy::multiple_bound_locations". We use multiple bounds as a workaround for an issue in pin-project-lite where only one bound can be assigned per location. The phrasing of pin-project-lite's README implies that this issue is WONTFIX. To quote:

This library does not tackle as expansive of a range of use cases as pin-project does. If your use case is not already covered, please use pin-project.

[notgull]

MIRI failure is tracked in https://github.com/smol-rs/event-listener/pull/133