Implement str.count and add tests for it.

Also modify mp_get_index to accept:
1. Indices that are or evaluate to a boolean.
2. Slice indices.
Add tests for these two cases.

tests/basics/list_index.py

@@ -3,6 +3,16 @@ print(a.index(1))
 print(a.index(2))
 print(a.index(3))
 print(a.index(3, 2))
+print(a.index(1, -100))
+print(a.index(1, False))
+
+try:
+    print(a.index(1, True))
+except ValueError:
+    print("Raised ValueError")
+else:
+    print("Did not raise ValueError")
+
 try:
     print(a.index(3, 2, 2))
 except ValueError: