src: refactor `BaseObject` internal field management

- Instead of storing a pointer whose type refers to the specific
  subclass of `BaseObject`, just store a `BaseObject*` directly.
  This means in particular that one can cast to classes along
  the way of the inheritance chain without issues, and that
  `BaseObject*` no longer needs to be the first superclass
  in the case of multiple inheritance.

  In particular, this renders hack-y solutions to this problem (like
  ddc19be6) obsolete and addresses
  a `TODO` comment of mine.

- Move wrapping/unwrapping methods to the `BaseObject` class.
  We use these almost exclusively for `BaseObject`s, and I hope
  that this gives a better idea of how (and for what) these are used
  in our code.

- Perform initialization/deinitialization of the internal field
  in the `BaseObject*` constructor/destructor. This makes the code
  a bit more obviously correct, avoids explicit calls for this
  in subclass constructors, and in particular ...