object: __tostring should call __totable metamethod, not method.

* lib/std/object.lua (new.__tostring): Because we have to set a
separate metatable for objects that have __lt or __le metamethods,
self:__totable () is not always the same as getmetatable (self).
__totable (self).  Use the longhand so that __tostring calls the
right function in both cases.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>