First cut of HAL Timer API

[will sanfilippo <wi...@runtime.io>]

Hello:

I have just committed the first cut of the HAL Timer API. There were some changes to the original email; most notably the removal of the helper functions to convert ticks to usecs/usecs to ticks.

I also committed an initial cut of this API implemented on the nrf52. After writing this I realized that there is something a bit confusing about the API but I didnt want to change it from the initial email. The issue is that there is a set of API that basically works on the underlying HW timer and a set of API that operates on a user-defined timer structure. It is not obvious from the name of the API which you are operating upon.

If folks are really bothered by this it can be easily changed.

Thanks!