This is not comprehensive, since in Python one can't determine what is
and what is not mutable; but I've added a few base cases (list, dict,
set).

The patch also improves (makes more uniform) the error messages in the
parameter definitions.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>