Support chained assignment statements, e.g. a = b = c.

We know when we have begun a chained assignment when we process a DUP_TOP with non-null on the stack. Push a NODE_CHAINSTORE onto the stack when this happens, and keep it 'floating' on top of the stack for all STORE_X operations until the stack is empty.
To support versions of Python <= 2.5 which use DUP_TOP in more places, I modified ROT_TWO, ROT_THREE and ROT_FOUR to get rid of NODE_CHAINSTORE on the stack if it is present.

tests/input/chain_assignment.py

@@ -0,0 +1,39 @@
+a = [y, z] = x = (k1, k2, k3) = [] = c = myfunc(x) + 3
+
+x = y = g = {keyA: X} 
+
+global store_global
+Gx = Gy = Gz = Gq1
+Gx = [Gy, Gz] = Gq2
+a = b = store_global = c
+
+def func_with_global():
+    global Gx, Gy, Gz, Gq
+    Gx = Gy = Gz = Gq
+
+y = store_subscr[0] = x
+a[0] = b[x] = c[3] = D[4]
+a[0] = (b[x], c[3]) = D[4]
+a[0] = Q = [b[x], c[3]] = F = D[4]
+q = v = arr[a:b:c] = x
+
+class store_attr1:
+    def __init__(self, a,b,c):
+        self.a = self.b = self.c = x
+        self.d = y
+
+class store_attr2:
+    def __init__(self, a,b,c): self.a = (self.b, self.c) = x
+
+a.b = c.d = e.f + g.h
+
+def store_deref():
+    a = I
+    a = b = c = R1
+    a = (b, c) = R2
+    def store_fast():
+        x = a
+        y = b
+        z = c
+        p = q = r = s
+        p = [q, r] = s