Improve and use unreachable

[privat]

unreachable is a hint to the developer and the compiler that the code is not reachable.

A VMClass>>unreachable method exists but 1. was not used that much, 2. was not a noop on production.
Lot of places used a comment "NOTREACHED" to indicate the same thing, but such comments are not checked at runtime in debug mode and they do not provide opportunities for optimizations.

So the PR improve unreachable and use it instead of "NOTREACHED" comments.

The only one that remain is in RegisterAllocatingCogit >> genJumpIf:to: and is inside a statement list, and that's weird.

A future improvement will be to pass the information to the C compiler (e.g. __builtin_unreachable in GCC, or unreachable in C23). But I'm still not sure about the best portable way to do that.

[guillep]

I like it!!!

CI was green on mac and windows, there was an issue in linux but not related.

[guillep]

Thanks! Why did you choose to use an assertion instead of an error?

[privat]

Thanks! Why did you choose to use an assertion instead of an error?

error generate code in release mode, assertion generate nothing so the C compiler can have better optimization (unlikely to be noticeable but I might be pessimistic)