Abstract Virtual Machine Support in byterun

I used byterun as the basis for an abstract VM for some work here at Google and I wanted to offer the changes back to you. This also fixes some bug, notably classes work correctly after the first commit. However the bulk of the changes are to allow abstract interpretation to be done.

-Arthur Peters (working from inside Google)
Because I copied the MRO algorithm code directly from the Python documentation which is under this license.