lock_api: Add default "atomic_usize" feature

[rmsyn]

Adds a default "atomic_usize" feature as a feature gate for library components that rely on AtomicUsize. Some platforms, such as AVR, do not support atomics with large widths.

Current library users will experience no changes, but users on constrained platforms can opt-out by using default_features = false.

[rmsyn]

Looks like the Windows CI runner timed out:

 The process 'C:\Program Files\Git\bin\git.exe' failed with exit code 128
  Waiting 13 seconds before trying again
  "C:\Program Files\Git\bin\git.exe" -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +5dee459357ae3328b5c2cbc8cc1a60527e1e344f:refs/remotes/pull/380/merge
  Error: fatal: unable to access 'https://github.com/Amanieu/parking_lot/': Failed to connect to github.com port 443 after 21047 ms: Couldn't connect to server
  Error: The process 'C:\Program Files\Git\bin\git.exe' failed with exit code 128