In what Haskell to Python opcodes are concerned, a Haskell 'Set' is
translated into a Python 'list'.  In other words, currently, opcodes
that handle sets of parameters are actually handling lists because
this is how sets are currently encoded.  This patch introduces a new
type called 'ListSet' that wraps a Haskell 'Set' and it is used to
represent on the Haskell side a Python 'list' without duplicate
elements.  This patch also updates the respective opcode parameters
and updates the opcode tests.

Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>