Add the async_run macro

[aedrax]

I like how you have the await macro built, but it wasn't usable from a synchronous function like main, so I made this alternative. I know it isn't super necessary, but I really like the way it pulls together with the rest of the async.h project

[naasking]

Sorry for the lateness here, I only just recently had enough time to start getting back into this stuff. My only concern with this macro is that it blocks due to the hidden loop, so someone could inadvertently call it from inside an async function thinking it will implicitly yield. That's why all current async_* macros have no blocking semantics. Minor nitpick maybe, but the while loop doesn't add much code anyway so I'm not totally convinced it's needed.

A name that makes this very clear might be acceptable:

#define block_until(cond) while (!(cond))

Then it's maybe a little clearer what's going on:

block_until( driver_thread(&driver_pt) == ASYNC_DONE );