Returns an array of the filenames of the objects that `ob` inherits
directly -- its immediate inherits only, one level deep. Each filename
carries a leading slash. If `ob` is omitted it defaults to
this_object().
Only the inherits named in `ob`'s own program are reported; the programs
those parents inherit in turn are not followed. To recurse through the
entire inheritance graph and list every inherited file at every depth,
use deep_inherit_list() instead.
The efun inherit_list() is an alias for shallow_inherit_list().