Big VM cleanup

Big VM cleanup
- Move forward declarations of 'struct factor_vm' to one place
- Rename template parameters from T and TYPE to descriptive names. New convention: CamelCase for template parameters
- Change some higher-order functions taking function pointers into templates, and define classes overriding operator(). There's a bit of new boilerplate here but its more consistent than the old mish-mash approaches
- Put GC state into a gc_state struct
- Use exceptions instead of longjmp for non-local control transfer in GC
- In code GC, instead of interleaving code block tracing with copying, add code blocks which need to be revisited to an std::set stored in the gc_state