Add support to access the current metaobject within a metaobject activation

[charig]

Currently, when executing a metaobject method for redefining VM behavior, the receiver is the receiver of the original operation. This means that the current metaobject is lost. There are a couple of alternatives for accessing the metaobject:

1. Add the metaobject as a special argument to all metaobject activations.
2. Add primitives to the language for accessing the current metaobject in a frame and the metaobject of an object.