Executable dispatch/suspension; code cleanup

Arpk::Executable can now track their suspension state (actual
suspension, however, is specific to the implementing class).  The
purpose of this is to permit background processes to be forced to idle
if necessary (such as to prevent races/collisions in data modification
during load).  As such, it is recommended that any Executable's runtime
loop sleep while #suspended?() returns true.

Executable instances may also be dispatched directly to the Thread Pool;
if one is detected, its #get_runtime() method will be called
automatically.