show() of nullary terms omits parens

[Matt Giuca]

type Nullary:
    Null()

?> Null
<constructor function Null>
?> Null()
Null

Expected result:
?> Null()
Null()

Printing "Null" implies that the expression Null evaluates to the same value, which it does not. Empty parens are required in the output.

This is a deficiency in the val_term constructor of interpret -- it cannot distinguish between parameterless and nullary terms. Change the list to a maybe(list), and remove the special case for the empty list when displaying.

[Matt Giuca (mgiuca)]

Fixed in trunk r1086.