Fix irregular switch labels so ordering is consecutive

[zbanks]

Would be nice to clean this up so ordering is consecutive, but it seems like low priority.

Originally posted by @simonlindholm in #180 (comment)

Also fix the issue where one switch may have index 0 (and therefore no comment) even if other switches are detected later.

[NWPlayer123]

If this is implemented, it should be a toggle. The ordering of switch cases will change codegen, the "correct" ordering is going by what's in the body of the switch and then outputting which cases reach that part of the body.

[simonlindholm]

This issue isn't about reordering switch cases (that would be very dangerous indeed, and we haven't found a compiler for which we need to do that), but rather about the indexing of // switch N comments, so that it goes N = 0, 1, ... and doesn't skip numbers.