Serialization opt-in syntax (summary)

[Remi Forax]

We have several choices:
  - use an annotation like @Serial, but currently there is no way o 
attach an annotation
     to an expression, so this was rule out.
  - use a cast to (Foo & Serializable) but the solution to allow users 
to use & to specify a type
    is far from easy and require investigation and we haven't time for that.

so if no one has a better idea, this means we need a ad-hoc syntax to 
specify that a lambda is serialize
(David, users want to serialize capturing lambda).

I think we have rule out to soon the fact that we also can use a 
specific syntax, perhaps not x ~> x or
x -s> x (the snake arrow Josh had proposed) but a specific syntax is a 
ad-hoc solution too.

Rémi