refactor(runtime): simplify TimerRuntime

[AsakuraMizu]

Background

I borrowed the implementation of compio's TimerRuntime in one of my projects. It has now been suggested that this implementation could be simplified (Starry-OS/arceos#20). Since I wasn't sure, I moved his simplified implementation back to compio so that the original author could take a look at it to see if there were any problems.

Related commits:

• 094d35d

[Copilot]

Pull request overview

This PR simplifies the TimerRuntime implementation by removing the FutureState enum and directly storing Waker instances in the timer wheel. The refactoring reduces complexity while maintaining the same functionality.

Key changes:

• Removed FutureState enum wrapper, storing Waker directly in BTreeMap<TimerKey, Waker>
• Changed remove_completed() to is_completed() with simpler semantics (checking absence from map)
• Refactored wake() method to use split_off() for efficient separation of expired vs pending timers

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
compio-runtime/src/runtime/time.rs	Removed FutureState enum, simplified TimerRuntime to store wakers directly, refactored wake() using split_off(), reordered struct field initialization to match declaration order
compio-runtime/src/runtime/mod.rs	Updated poll_timer to use renamed is_completed() method

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