Remove the dependency on async-lock

[james7132]

This PR removes the need for async-lock as a dependency by making the Executor initializable in a const context. This relies on the const initialization of CondVars and Mutexes, so the MSRV was bumped to 1.68.

This MSRV bump may not be acceptable, so either this PR must wait, or an alternative queue implementation needs to be chosen.

[notgull]

As an organization we maintain an MSRV policy where the MSRV should not exceed Debian Stable's packaged version of rustc. In this case this is 1.63.0. So we need to wait before this can be merged.

[taiki-e]

This relies on the const initialization of CondVars and Mutexes, so the MSRV was bumped to 1.68.

(const Mutex::new and const Condvar::new are available since 1.63. That actually requires 1.68 is const VecDeque::new)

[james7132]

Found a way to lower the MSRV to 1.63 by using Option<VecDeque<T>> instead. This shouldn't need extra branches due to the niche layout optimization of VecDeque and Option.

[notgull]

Thanks!