Support for big numbers

[pliniker]

Numbers that overflow (size_of::<isize>() * 8) - 2 bits should be stored in NumberObject objects.

I have no experience implementing number objects and I understand that there may be commonly used or standard schemes and/or libraries? Needs an expert.

Depends on #22

• implement NumberObject (we can rename this)
• extend VM arithmetic opcodes to handle number objects
• extend book chapter from #22