Add support for higher order functions

[WenqingZong]

The current implementation does not work with higher order functions.

Minimal code to reproduce:

from crab_dbg import dbg

def higher_order_function(func, *args, **kwargs):
    func(*args, **kwargs)

pai = 3.14
dbg(pai)  # This works
higher_order_function(dbg, pai)  # This does not work