Task isn't started yet, execute it in the current thread.
If it's done, return its return value, if any. If it's in progress,
busy spin until it's done, then return the return value. If it threw
an exception, rethrow that exception.
. ReturnType spinForce() @property @trusted;
This function should be used when you expect the result of the
Task to be available on a timescale shorter than that of an OS