Add mutability quantification

[stephanemagnenat]

Ferlium currently does not support mutability quantification, and does eager mutability defaulting. This prevent properly expressing the following function:

fn f(a, g) {
    g(a)
}

In that case the type of f is: f<A, B, e>(a: A, g: (A) -> B ! e) -> B ! e which has defaulted the mutability of a to constant. However, the proper/most generic type scheme should be: f<A, B, m, e>(a: &m? A, g: (&m? A) -> B ! e) -> B ! e.

[stephanemagnenat]

The key issue is that this would make functions generic over their calling conventions, we probably do not want that.