Optional members of record constructors

[AshleyYakeley]

Something like:

datatype T of
    MkT of
        m: X = d;
    end;
end;

[AshleyYakeley]

Still to do:

• [x] documentation
• [x] get doc autogen to use it
• [x] make use of in UILib

[AshleyYakeley]

Done.