Improve vm perf

[jgaskins]

Made a couple small tweaks to the VM to improve performance. Numbers running it on an M1 MacBook Pro:

main

➜  monyet git:(main) ./build.sh && ./benchmarks.sh --vm-fast
-----8<------
"engine=vm, 9227465, duration=00:00:03.700458291"

This PR

➜  monyet git:(improve-vm-perf) ./build.sh && ./benchmarks.sh --vm-fast
-----8<------
"engine=vm, 9227465, duration=00:00:03.042388958"

… which makes it faster than the Go implementation on the same machine:

➜  monkey git:(main) ./fibonacci
engine=vm, algo=fast, result=9227465, duration=3.101420333s

How did we get here?

• Memoized current_frame rather than computing it each time, avoiding many bounds checks on both integer addition and array indexing. This was the single largest contributor to performance improvement.
• For other integer addition, bypass bounds checking on things like instruction pointers — the stack would have to exceed 2GB for this to be a problem and I don't think that's supported by many programming languages

One other change that didn't seem to do much for performance but that I left in anyway was to replace a few case self with plain-old type inheritance.

[jgaskins]

Add the numbers from my Intel Mac:

main

➜  monyet git:(main) ./build.sh && ./benchmarks.sh --vm-fast

"engine=vm, 9227465, duration=00:00:04.346747996"

This PR

➜  monyet git:(improve-vm-perf) ./build.sh && ./benchmarks.sh --vm-fast

"engine=vm, 9227465, duration=00:00:03.586341877"

Go implementation

➜  monkey git:(main) ./fibonacci
engine=vm, algo=fast, result=9227465, duration=3.716671473s

[MarioAriasC]

Thanks I'll have a look