Fix how functions are parsed for const exprs

[ppotapov-aws]

Allows tracing kernels like:

def func():
  print("Foo")

class FuncWrapper(nl.NKIObject):
    d = func

def kernel():
    FuncWrapper.d()  # used to be a problem. We parsed `d` as a ref to function object, instead a function call
    fw = FuncWrapper()
    fw.d() # still works as expected