more robust shadowing technique

[jaberg]

The technique of shadowing temporaries by id was a good start, but not robust. Problems are:

• it has to disable gc (not a big deal)
• it is confused by a single object showing up twice by coincidence (especially small integers!)

What would make this better is a proper virtual machine class (with globals, locals, and a stack) that is instantiated once to run the original program, and once to run the symbolic shadow program.