`Scheduler`s are no longer passed as implicit arguments.
Docs fix.
Add scheduler bundles.
Fuse scheduler helper objects with scheduling conditions.

For example, `Runnable` used by the `Executor` scheduler now contains
the fields `interruptRequested` and `allowedBudget`, which are removed
from the `IsolateFrame`.

This allows implementing schedulers more flexibly.
Moving isolate initialization code to the base `IsolateSystem` class.