thread - Thread abstraction
TODO
The thread object is the construct that represents a time-shared CPU execution context. Thread objects live associated to a particular Process Object which provides the memory and the handles to other objects necessary for I/O and computation.
A thread can be created implicitly by calling sys_process_start(), in which
case the new thread is the "main thread" and the thread entrypoint is defined by
the previously loaded binary. Or it can be created by calling
sys_thread_create() which takes the entrypoint as a parameter.
A thread terminates when it returns from executing the routine specified as
the entrypoint or by calling sys_thread_exit().
TODO (add links to thread syscall docs)