Macro-processor: implement comprehensions

Due to a limitation of the current implementation, this breaks syntaxes like
[ (i,j) ] (but not [ (2,j) ]; the problem only occurs when an array is
constructed by specifying as first element a tuple whose first element is a
variable name). Solving this problem requires an overhaul of the
macro-processor, with construction of ASTs at parsing time, and evaluation
later on (instead of doing on-the-fly evaluation).

Ref #5