Add a `Timer` type/trait

[PTaylor-us]

I think it's important to make a distinction between a clock and a timer. These are my current working definitions:

• Clock: Any entity that periodically counts (ie a hardware timer/counter peripheral). Generally, this needs to be monotonic. A wrapping clock is considered monotonic in this context as long as it fulfills the other requirements.
• Timer: An entity that counts toward an expiration.

In terms of a Clock interrupt/wake feature I was working on, a timer is based on a clock and holds the instant at which the timer expires. Upon expiration a number of different things could happen including an interrupt/wake, a semaphore, etc.

Features I would expect for a clock:

• get an instant

Features I would expect from a timer:

• set the duration
• start
• restart (not resume)
• poll the status (not-running, running-enabled, running-disabled, expired)
• set the action to be taken upon expiration
• enable/disable action (enable/disable interrupt)
• block until expired (wait/delay)