implement timer module

[hyperthunk]

so we can

• sendAfter
• runAfter
• tick
• cancelTimer

[hyperthunk]

see https://github.com/hyperthunk/distributed-process-platform/tree/timer for an initial sketch.

[hyperthunk]

This issue will remain open until the branch is fully merged.

[hyperthunk]

This issue depends on #18 - bar the potential race in both cancelTimer and flushTimer were mostly done.

[hyperthunk]

Now issue #18 is resolved and the Timer module was merged ages ago => resolved.