Fix some issues in the schedular

[DelSkayn]

This PR fixes some problems with the schedular.

• Fixes a issue where rquickjs spawned futures could outlive the context resulting in an abort.
• Fixes an issue where idle could miss futures returning early.
• Fixes the futures returned from idle and execute_pending_job not being send/sync when parallel was enabled. Fixes #276
• Removes an unneeded assertion which would sometimes trigger.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.06061% with 46 lines in your changes are missing coverage. Please review.

Project coverage is 68.47%. Comparing base (84d50f3) to head (56041ef).

Files	Patch %	Lines
core/src/runtime/async.rs	68.90%	37 Missing :warning:
core/src/util.rs	80.64%	6 Missing :warning:
core/src/context/async/future.rs	94.44%	1 Missing :warning:
core/src/runtime/schedular.rs	98.33%	1 Missing :warning:
core/src/runtime/schedular/waker.rs	94.73%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #302 +/- ## ========================================== + Coverage 68.06% 68.47% +0.41% ========================================== Files 81 83 +2 Lines 12214 12375 +161 ========================================== + Hits 8313 8474 +161 Misses 3901 3901 ```

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