timers: enable timers to be used as primitives

For web compatibility this allows timers to be stored as the key of an Object property and be passed back to corresponding method to clear the timer.

Note that this is using the async_id_symbol to get a numeric ID to correspond with timers.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines