v2.8.0

[notgull]

• Fix a bug where the SemaphoreGuard::acquire_arc future would busy wait under certain conditions (don't busy wait in SemaphoreGuard::acquire_arc #42).
• Add a Semaphore::add_permits() function to increase the number of available permits on the semaphore (Add Semaphore::add_permits() #44).
• Make RwLockReadGuard covariant over its lifetime (Make RwLockReadGuard covariant #45)
• Add RwLockReadGuardArc, RwLockWriteGuardArc, and other reference counted guards for the RwLock type (Add reference-counted versions of RwLock guards #47).
• Loosen the Send/Sync bounds on certain future types (Loosen Send/Sync bounds on futures #48).
• Fix UB caused by the MutexGuardArc::source function allowing the user to drop an object in a different thread than the one it was acquired in (Fix UB in MutexGuardArc::source #50). This is a breaking change, but in the name of soundness. Therefore it doesn't break any valid behavior.
• Fix a bug where this crate would not compile properly on wasm64 (Replace target_os = "wasm64" with target_arch = "wasm64" #51).

[taiki-e]

Thanks!