py: Implement break and continue byte codes, and add tests.

Also fixes a bug in the for-in-range optimiser.

I hope to remove break and continue byte codes in the future and just
use jump (if possible).

tests/basics/continue.py

@@ -0,0 +1,16 @@
+for i in range(4):
+    print('one', i)
+    if i > 2:
+        continue
+    print('two', i)
+
+for i in range(4):
+    print('one', i)
+    if i < 2:
+        continue
+    print('two', i)
+
+for i in [1, 2, 3, 4]:
+    if i == 3:
+        continue
+    print(i)