Fix type instabilities in FunctionCallingCallback

[albertomercurio]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

The previous implementation of FunctionCallingCallback was type unstable, because the type of the funcat_internal variable was depending from the value of tdir.

Here I fix this by removing the BinaryMinHeap and BinaryMaxHeap usage.