Add a high-resolution real-time clock #17

Clock: Add ability to convert from microseconds to kernel time ...

This functionality allows us to ask the kernel to schedule events
based on a high resolution real-time clock.

An example in main waits until the next second.

Clock: Combine clocks to create a high resolution real-time clock ...

The kernel clock is high resolution but only tracks uptime.
The RTC clock is low resolution but tracks real time.
The NTP clock is high resolution but costly to access.

We can combine the kernel clock with a real-time clock of any type
by sporadically calculating the time the system was booted at then
adding the kernel's uptime to it whenever we need a new clock.

Clock: Clean up and document the code

Clock: Use microseconds instead of struct timeval ...

timevals are a little annoying to work with compared to just microseconds,
so switch to using them for our clock API.

Clock: Print microseconds in ISO 8601 strings

Clock: Add _now suffix and variants for clock functions ...

Currently the clock code implicitly grabs the current time.
Add variants that let you pass in your own time value, and specify
extra _now functions that use the current time.

CMakeLists: Apply compile options to Mbed too ...

This was my original intention but I guess I had things in the wrong order.