[Language] allow to initialize fields in `class` and `struct` bodies

[igotfr]

in most (if not all) programming languages with class or struct is allowed to initialize a field directly at declaration in its block, something like:

class Cl(a: u8, b: u8) {
  c: u8 = 4
}

async fn main() {
  let cl: Cl = Cl{1, 2}
  cl.c # access 4
}

in Dada, when I try it I get :

c: u8 = 4
  |         ^ I don't know what to do with this, it appears to be extra

Examples of programming languages that works like this: Swift, Kotlin, C++, Python, Java, C#, Dart, Javascript

It would allow too pass arguments to parameters similar to functions:

class Cl(a: u8, b: u8) : (p1: u8, p2: u8) {
  c: u8 = p1 + p2
}

async fn main() {
  let cl: Cl = Cl{1, 2}(3, 4)
  cl.c # access c, which is p1 + p2, in this case: 7

  cl.p1 # error, p1 is not a field
}

Examples of programming languages that works like this: Kotlin, Dart

this proposal allow to initialize fields this way

Correlated to

#262