Implement support for sys.path when loading modules.

sys.path is not initialized by rt_init(), that's left for platform-specific
startup code. (For example, bare metal port may have some hardcoded defaults,
and let user change sys.path directly; while port for OS with environment
feature can take path from environment). If it's not explicitly initialized,
modules will be imported only from a current directory.

py/qstrdefs.h

@@ -73,6 +73,7 @@ Q(max)
 Q(min)
 Q(next)
 Q(ord)
+Q(path)
 Q(pow)
 Q(print)
 Q(range)