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.
diff --git a/py/objtuple.c b/py/objtuple.c
index d39b36d..827441f 100644
--- a/py/objtuple.c
+++ b/py/objtuple.c
@@ -111,7 +111,7 @@
                 return res;
             }
 #endif
-            uint index = mp_get_index(o->base.type, o->len, rhs);
+            uint index = mp_get_index(o->base.type, o->len, rhs, false);
             return o->items[index];
         }
         case RT_BINARY_OP_ADD: