tests/bench/var: Add tests for class/instance var access.

Also compared with method abstraction for accessing instance vars -
it's more than 3 times slower than accessing var directly.

tests/bench/var-5-class-attr.py

+import bench
+
+class Foo:
+    num = 20000000
+
+def test(num):
+    i = 0
+    while i < Foo.num:
+        i += 1
+
+bench.run(test)

tests/bench/var-6-instance-attr.py

+import bench
+
+class Foo:
+
+    def __init__(self):
+        self.num = 20000000
+
+def test(num):
+    o = Foo()
+    i = 0
+    while i < o.num:
+        i += 1
+
+bench.run(test)

tests/bench/var-7-instance-meth.py

+import bench
+
+class Foo:
+
+    def __init__(self):
+        self._num = 20000000
+
+    def num(self):
+        return self._num
+
+def test(num):
+    o = Foo()
+    i = 0
+    while i < o.num():
+        i += 1
+
+bench.run(test)