Add keeping track of slot numbers for vtable entries and emit placeholders when they're removed

[PathogenDavid]

Right now it isn't possible to safely remove a vtable entry in a transformation because vtables are emitted as sequential structs. We should keep track of slot numbers and ensure we emit a padding field when one is missing.

[PathogenDavid]

~~Instead of keeping track of slot numbers, let's change TranslatedVTableEntry to refer to the TranslatedFunction it corresponds to using https://github.com/InfectedLibraries/Biohazrd/issues/150 instead.~~ This issue is actually about something different than I originally thought.

[PathogenDavid]

Note: This is not what PathogenVTableEntry.Offset provides. That's specifically only relevant for VCallOffset, VBaseOffset, and OffsetToTop entries.

[PathogenDavid]

We can fudge this without it, but it'd be a good idea to do this at the same time as https://github.com/InfectedLibraries/Biohazrd/issues/49