Mv Error into coro.zig

Add section about switching to Zig async/await

Update asyncio module for task-like execution (rsepassi#6)

Add top-level run for coroutine kick-off.
Add asyncio.xawait for parallel execution.
Store everything but the return value in the coro stack.
Ensure coroutines have a stable parent that's yielded to on
return.
Remove unnecessary constCast in coro storage.
Clean up aio tests.
Update readme

Add async IO with libxev (rsepassi#5)

Add license