Sync Coroutine Support

[kammce]

This is an epic for collecting all of the utilities for supporting synchronous coroutines. The purpose of supporting sync coroutine calls, is that we will have clients that do not want to interact with the coroutine code. They want their code to be as simple as:

auto led = get_led(); // strong_ptr<hal::output_pin>
led->level(true); // turn on
hal::delay(clock, 100ms);
led->level(false); // turn off

They don't want to deal with the async context and such. So for them we can provide wrappers and a global async context that each sync wrapper can use "per thread". Needs to be per thread because threads cannot share the sync context as that would result in race conditions galore.