Implement __bool__ and __len__ via unary_op virtual method for all types.

__bool__() and __len__() are just the same as __neg__() or __invert__(),
and require efficient dispatching implementation (not requiring search/lookup).
type->unary_op() is just the right choice for this short of adding
standalone virtual method(s) to already big mp_obj_type_t structure.

tests/basics/true-value.py

@@ -3,9 +3,18 @@
 if not False:
     print("False")
 
+if not None:
+    print("None")
+
 if not 0:
     print("0")
 
+if not 0.0:
+    print("float 0")
+
+if not 0+0j:
+    print("complex 0")
+
 if not "":
     print("Empty string")
 if "foo":