Separate type of node signature from function value

[acl-cqc]

At present we use FunctionType for both, but there is (following #787) an important difference - node signatures cannot contain row variables, whereas types of function values can. In particular, we should probably not talk about "ports" of such function values (the various methods on FunctionType) as the indices of elements in these TypeRows can change when the row variables are substituted.

[doug-q]

closed by #1138