Issue1586

It seems like some styles of function invocation might leak a strong reference to the function arguments.  Given a 

  def make_ref(obj, kwarg=True):
      return weakref.ref(obj, callback)

Calling make_ref(someobj) returns a weakref that functions as expected, but make_ref(someobj, kwarg=True) returns a weakref that seemingly can't ever go stale.  Test case attached.

This is the fault of the compiler. Function calls w/ keywords create a PyObject[] positional+keyword args array and stores it in a local variable (of the JVM stack). That's supposed to be for temporary storage but the compiler doesn't explicitly null the local variable after the call is made, so the reference lingers until the Java method finishes

If you need to workaround this problem before we fix it, these reference should be freed when the current Python function returns

In similar usage in the compiler to visitCall, we explicitly null out using the CodeCompiler#freeArray (such as visitList, visitDict, etc).

Fixed by r7047