done bits

* fix all the ``*-into`` functions/methods to take the destination
  variable name as 1st argument (otherwise optional arguments become a
  mess)

* expose namespace open/close, and ``namespace $name; do …; done``

  - use them inside ``class_open`` / ``class_close``

* ``bos/mop/base`` + ``bos/mop/inheritance`` become ``bos/meta/class``

  * store class→metaclass association somewhere (global dictionary,
    probably)

  - when creating a class, create a metaclass instance and store it
  - the store must be pre-populated with metaclass instances for:

    + ``bos/meta/class`` → ``bos-dispatch/invoke bos/meta/class 1``

* add ``bos/meta/attribute``

  - store values in ``bos-namespaces/store-*-for-into attribute
    $attrname $defining_class $object_class $object_id`` (notice, we
    also need the class that owns the attribute, not just the class
    that generated the object, because attributes are local to the
    class, they're not inherited!)

  - ``get-value`` / ``set-value`` methods
  - add name, type, owning metaclass as attributes to itself

next bits…

* add attribute support to ``bos/meta/class``

  - ``find-attribute-into``
  - ``add-attribute``
  - add class, isa, mro, as attributes to the metaclass

  maybe done? very untested

* pre-populated metaclass instances for:

  - ``bos/meta/class`` → ``bos-dispatch/invoke bos/meta/class 1``
  - ``bos/meta/attribute`` → ``bos-dispatch/invoke bos/meta/class 2``

  and those instances' attributes must be manually stored

  setting up the isa/mro for those two is going to be *complicated*

* constructor should assign attributes

  - this will make ``has`` much simpler to write!

* formalise that ``bos-dispatch/invoke $class 0 $method $args`` is a
  class method invocation