Representation Types.

One of the underspecified corner in S12 is how class objects (such as ::Array, ::Int and ::Class) are represented internally, how those representation differs from the p6opaque layout, and how different representations interact.

Today Stevan's representation types design addressed these issues, and we figured out how to cleanly support Larry's class objects are like undefined instance objects intuition via the nil reprensetation type. Eigenclasses and prototype-based systems are also very straightforward under this design. We'll make it happen over this weekend for sure. Woot!