oceani: Make 'List' separate from Print

Create a stand-alone ExpressionList which uses the List binode rather
than the Print binode.

The Print statement no longer uses a NULL entry on the end of the list
to denode a trailing comma.  Rather ->left is used for a normal print
list and ->right is used for a print list that has a trailing comma.

Signed-off-by: NeilBrown <neil@brown.name>