Merge: model: group parameter types in `MSignature::to_s`

Modify `MSignature::to_s` to make its output a shorter valid Nit signature. This helps readability and shortens error messages allowing them to fit more easily on a single line or within a text editor.

Example program:
~~~nit
import geometry
var b = new Box3d[Float] # <- error
~~~

Previous error message:
~~~
Error: expected 6 argument(s) for `init(left: Float, right: Float, top: Float, bottom: Float, front: Float, back: Float)`; got 0. See introduction at `core::Object::init`.
~~~

New error message:
~~~
Error: expected 6 argument(s) for `init(left, right, top, bottom, front, back: Float)`; got 0. See introduction at `core::Object::init`.
~~~

Pull-Request: #2529