Previous to this patch a call such as list.append(1, 2) would lead to a
seg fault.  This is because list.append is a builtin method and the first
argument to such methods is always assumed to have the correct type.

Now, when a builtin method is extracted like this it is wrapped in a
checker object which checks the the type of the first argument before
calling the builtin function.

This feature is contrelled by MICROPY_BUILTIN_METHOD_CHECK_SELF_ARG and
is enabled by default.

See issue #1216.