'interaction' in `design-matrix` should create raw columns as well

[behrica]

Currently (after implementing #41 ) we needed to write:

 [
 :cyl
 :disp
 [:cyl:disp '(+ :cyl :disp)]
]

To get three columns in the desing-matrix fn.

R does this automatically, so maybe we want the same, so that:

 [
 [:cyl:disp '(+ :cyl :disp)]
]

creates as well three columns (:cyl :disp :cyl:disp)

But then we need as well 'syntax` to suppress them, if not wanted.