This enables tracking time intervals of more than a couple seconds on
modern processors.

In extend=true, the reload interrupt is not disabled (for overflow
detection), the previous `set_compare()` may have been called with a
short interval, then, if the queue is empty, the reload interrupt would
fire at very short interval.

Also add comments clarifying the now() calls

* Move the SysTick clock source configuration to `new()`.
* Add comments about the double reset of the cycle counter.

depend on cortex-m 0.7.4 for `DWT::cycle_count()`

Co-authored-by: Henrik Tjäder <[email protected]>