[GR-64208] Reduce virtual calls to Object and reference visitors.

[graalvmbot]

• Call reference visitors once per sequence of references (instead of once per references). This reduces the number of virtual calls significantly.
• Visitors no longer return a boolean value to determine if visiting should continue. In the rare case that a visitor wants to stop visiting, it needs to throw an exception.
• Use a signed value for the count in InstanceReferenceMap.
• Prefer Pointer.toObjectNonNull() over Pointer.toObject().
• Remove support for derived references.
• Remove PathExhibitor.