py/objdict: Implement dictionary union (PEP 584).

Implements dictionary union according to PEP 584's specifications, minus
the fact that dictionary entries are not guaranteed to be in insertion
order.  This feature is enabled with MICROPY_CPYTHON_COMPAT.

Includes a new test.

With the assistance of Fangrui Qin <qinf@purdue.edu>

Signed-off-by: Rayane Chatrieux <rayane.chatrieux@gmail.com>
Signed-off-by: Damien George <damien@micropython.org>

tests/basics/dict_union.py

@@ -0,0 +1,36 @@
+# Test dictionary union.
+#
+# The tests sort the resulting dictionaries for now, since map insertion
+# order is not implemented in MicroPython.
+
+try:
+    {} | {}
+except TypeError:
+    print("SKIP")
+    raise SystemExit
+
+
+def print_sorted_dict(d):
+    print(sorted(d.items()))
+
+
+def test_union(a, b):
+    print_sorted_dict(a | b)
+    print_sorted_dict(b | a)
+    a |= a
+    print_sorted_dict(a)
+    a |= b
+    print_sorted_dict(a)
+
+
+d = {}
+e = {}
+test_union(d, e)
+
+d = {1: "apple"}
+e = {1: "cheese"}
+test_union(d, e)
+
+d = {"spam": 1, "eggs": 2, "cheese": 3}
+e = {"cheese": "cheddar", "aardvark": "Ethel"}
+test_union(d, e)

tests/basics/dict_union.py.exp

@@ -0,0 +1,12 @@
+[]
+[]
+[]
+[]
+[(1, 'cheese')]
+[(1, 'apple')]
+[(1, 'apple')]
+[(1, 'cheese')]
+[('aardvark', 'Ethel'), ('cheese', 'cheddar'), ('eggs', 2), ('spam', 1)]
+[('aardvark', 'Ethel'), ('cheese', 3), ('eggs', 2), ('spam', 1)]
+[('cheese', 3), ('eggs', 2), ('spam', 1)]
+[('aardvark', 'Ethel'), ('cheese', 'cheddar'), ('eggs', 2), ('spam', 1)]