added Object#rmext_ivar, shortcut to instance_variable_get/set
* Layout: Added #reopen to yield the instance to a block for further processing
* Layout: Added an internal "constraint table" to keep track of normalized equations and
  NSLayoutConstraint objects.  equations can now be modified simply by re-applying the same
  equation with a different constant.
* Layout: Added #xeq to remove a constraint by equation
* Layout: Added #remove(constraint(s)) to remove NSLayoutConstraint obects from the view
  and also from the internal constraint table.
bump