parametrize data types

[lucaferranti]

currently dual (and multidual) numbers hard-code to use real. It would be good to make the type a generic

struct dual {
  type t;
  var primPart: t;
  var dualPart: t;
}

this would have a few advantages:

• allow dual of dual, which would effectively mimic hyperdual numbers and give support for higher order derivatives
• allow to use dual number of custom types (after all, I mainly created this to eventually use it with my interval arithmetic library)

I had tried at some point but hit a few corners. Now that there's a working non-generic skeleton, it should be expanded.