Stacked runners: Add a futex and threads to runner processes

[Sangho Lee (sangho2)]

This PR adds a futex and threads to userland runner processes, which the kernel schedules and synchronizes. It also replaces the BrokerEnter kernel call with Futex, so a runner waits on and wakes the kernel broker the same way it does other threads.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/inherent_method_missing.ron

Failed in:
  Association::enter, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/017ee407e75568a4c9b675bea38d0756881cc24d/litebox_broker_vm_kernel/src/lib.rs:383

--- failure enum_missing: pub enum removed or renamed ---

Description:
A publicly-visible enum cannot be imported by its prior path. A `pub use` may have been removed, or the enum itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/enum_missing.ron

Failed in:
  enum litebox_common_vm_abi::BrokerEnterOp, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/017ee407e75568a4c9b675bea38d0756881cc24d/litebox_common_vm_abi/src/lib.rs:613

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/enum_variant_added.ron

Failed in:
  variant Request:Futex in /home/runner/work/litebox/litebox/litebox_common_vm_abi/src/lib.rs:343
  variant Request:Clone in /home/runner/work/litebox/litebox/litebox_common_vm_abi/src/lib.rs:343
  variant Request:ExitThread in /home/runner/work/litebox/litebox/litebox_common_vm_abi/src/lib.rs:343
  variant Request:InterruptThread in /home/runner/work/litebox/litebox/litebox_common_vm_abi/src/lib.rs:343
  variant Status:Interrupted in /home/runner/work/litebox/litebox/litebox_common_vm_abi/src/lib.rs:233
  variant CallId:Futex in /home/runner/work/litebox/litebox/litebox_common_vm_abi/src/lib.rs:343
  variant CallId:Clone in /home/runner/work/litebox/litebox/litebox_common_vm_abi/src/lib.rs:343
  variant CallId:ExitThread in /home/runner/work/litebox/litebox/litebox_common_vm_abi/src/lib.rs:343
  variant CallId:InterruptThread in /home/runner/work/litebox/litebox/litebox_common_vm_abi/src/lib.rs:343

--- failure enum_variant_missing: pub enum variant removed or renamed ---

Description:
A publicly-visible enum has at least one variant that is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/enum_variant_missing.ron

Failed in:
  variant Request::BrokerEnter, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/017ee407e75568a4c9b675bea38d0756881cc24d/litebox_common_vm_abi/src/lib.rs:292
  variant CallId::BrokerEnter, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/017ee407e75568a4c9b675bea38d0756881cc24d/litebox_common_vm_abi/src/lib.rs:292

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/struct_missing.ron

Failed in:
  struct litebox_common_vm_abi::BrokerEnterRequest, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/017ee407e75568a4c9b675bea38d0756881cc24d/litebox_common_vm_abi/src/lib.rs:628

--- failure trait_method_missing: pub trait method removed or renamed ---

Description:
A trait method is no longer callable, and may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-item-signature
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/trait_method_missing.ron

Failed in:
  method receive of trait NotificationReceiver, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/017ee407e75568a4c9b675bea38d0756881cc24d/litebox_platform_vm_userland/src/lib.rs:245