#3 Add support for general GF(2^p)

[Riccardo Bernardini]

Currently all the code in the Vandermonde profile is hard-tied to GF(2^32). We should add support also for GF(2^d), d=4, 8, 16. Note that the resulting code should be as efficient as possible. Maybe the easiest way is to make both disentangler and entangler generic and instantiate 4 different type of packages, at run-time we allocate the rigth type.