Previously, only type operators starting with ":" were type constructors,
and writing "+" in a type resulted in a type variable.  Now, type
variables are always ordinary identifiers, and all operators are treated
as constructors.  One can still write type variables in infix form though,
for example, "a `fun` b" is a type expression with 3 type variables: "a",
"fun", and "b".

Writing (+) in an import/export list always refers to the value (+)
and not the type.   To refer to the type one can write either "type (+)",
or provide an explicit suobrdinate list (e.g., "(+)()").  For clarity,
one can also combine the two, for example "type (+)(A,B,C)" is also
accepted and means the same thing as "(+)(A,B,C)" (i.e., export the type
(+), with the constructors A,B,and C).