py/runtime: Allow multiple **args in a function call.

This is a partial implementation of PEP 448 to allow multiple ** unpackings
when calling a function or method.

The compiler is modified to encode the argument as a None: obj key-value
pair (similar to how regular keyword arguments are encoded as str: obj
pairs).  The extra object that was pushed on the stack to hold a single **
unpacking object is no longer used and is removed.

The runtime is modified to decode this new format.

Signed-off-by: David Lechner <david@pybricks.com>

tests/basics/fun_calldblstar4.py

@@ -0,0 +1,33 @@
+# test calling a function with multiple **args
+
+
+def f(a, b=None, c=None):
+    print(a, b, c)
+
+
+f(**{"a": 1}, **{"b": 2})
+f(**{"a": 1}, **{"b": 2}, c=3)
+f(**{"a": 1}, b=2, **{"c": 3})
+
+try:
+    f(1, **{"b": 2}, **{"b": 3})
+except TypeError:
+    print("TypeError")
+
+# test calling a method with multiple **args
+
+
+class A:
+    def f(self, a, b=None, c=None):
+        print(a, b, c)
+
+
+a = A()
+a.f(**{"a": 1}, **{"b": 2})
+a.f(**{"a": 1}, **{"b": 2}, c=3)
+a.f(**{"a": 1}, b=2, **{"c": 3})
+
+try:
+    a.f(1, **{"b": 2}, **{"b": 3})
+except TypeError:
+    print("TypeError")