py: Remove DELETE_SUBSCR opcode, combine with STORE_SUBSCR.

This makes the runtime and object APIs more consistent.  mp_store_subscr
functionality now moved into objects (ie list and dict store_item).

tests/basics/del-attr.py

@@ -0,0 +1,28 @@
+# del a class attribute
+
+del C.f
+try:
+    print(C.x)
+except AttributeError:
+    print("AttributeError")
+try:
+    del C.f
+except AttributeError:
+    print("AttributeError")
+
+# del an instance attribute
+
+c = C()
+
+c.x = 1
+print(c.x)
+
+del c.x
+try:
+    print(c.x)
+except AttributeError:
+    print("AttributeError")
+try:
+    del c.x
+except AttributeError:
+    print("AttributeError")

tests/basics/del-name.py

@@ -0,0 +1,18 @@
+# del global
+
+x = 1
+print(x)
+del x
+try:
+    print(x)
+except NameError:
+    print("NameError")
+try:
+    del x
+except: # NameError:
+    # FIXME uPy returns KeyError for this
+    print("NameError")
+
+class C:
+    def f():
+        pass

tests/basics/del-subscr.py

@@ -0,0 +1,13 @@
+l = [1, 2, 3]
+print(l)
+del l[0]
+print(l)
+del l[-1]
+print(l)
+
+d = {1:2, 3:4, 5:6}
+del d[1]
+del d[3]
+print(d)
+del d[5]
+print(d)