Fix serious soundness error in JsProxy_Call kwargs handling

[hoodmane]

This resolves #974. There is a serious bug in JsProxy_Call:

  if (PyDict_Size(kwargs)) {
    JsRef idkwargs = python2js(kwargs);
    hiwire_push_array(idargs, idkwargs);
    hiwire_decref(idkwargs);
  }

"If there are no kwargs, the argument kwargs can be NULL or an empty dictionary." This code assumes it will be an empty dictionary but in practice, it's usually NULL (to avoid the overhead for making an empty dict).
The definition of PyDict_Size is:

Py_ssize_t
PyDict_Size(PyObject *mp)
{
    if (mp == NULL || !PyDict_Check(mp)) {
        PyErr_BadInternalCall();
        return -1;
    }
    return ((PyDictObject *)mp)->ma_used;
}

So we see that what usually happens is that it sets BadInternalCall and returns -1 indicating an error. As usual for this codebase, the calling code doesn't check for errors. What happens next is confusing to me. I think the next python API that is called is js2python ==> JsProxy_cnew ==> JsProxyType.tp_alloc (neither js2python nor JsProxy_cnew attempt any validation on their arguments). Chasing through tp_alloc to find where error handling happens is confusing, but on the master build, the error gets cleared. In various of my branches, I have added better error handling which causes the BadInternalCall to be correctly thrown.

The result of all of this is that most of the time javascript functions get called with an extra glitched argument.

function f(x){
    window.x = x;
}
pyodide.runPython("from js import f; f()");
x.toString() // ==>  "<NULL>"  (this is PyObject_Print(NULL, stdout, 0);)
x() // ==> Uncaught Error: TypeError: '�' object is not callable

Just typing x. into the browser console, the browser attempts to lookup the fields on x for intellisense and calls dir on the null pointer, so as I'm typing these examples the console is belching errors like this:

Python exception: pyodide.asm.js:8:504291
SystemError: frame does not exist

[hoodmane]

@rth This is ready for review.

[rth]

I don't have much review bandwidth this week so I may take a while to respond/review (particularly for more complex PRs)

[hoodmane]

Okay, thanks for letting me know.

[hoodmane]

@dalcde @rth I would much appreciate a review of this if either of you have time.

[hoodmane]

Just for fun, I posted supports_kwargs as a codegolf question:
https://codegolf.stackexchange.com/questions/217511/does-this-javascript-function-support-keyword-arguments

[rth]

Please add a changelog entry and maybe update the title to be more specific and mention kwargs.