Implement Instance.IsA and related methods

[Anaminus]

• Was skipped because descriptors weren't implemented yet.
• When the instance has a descriptor, methods work as normal.
• When the instance has no descriptor, methods just return the sensible negative (false, nil).
• The reflected methods could also return the negative, or could instead throw errors.
• Implement the following methods:
  • IsA
  • FindFirstAncestorWhichIsA
  • FindFirstChildWhichIsA