Infer if fields are mutated

[yannbolliger]

Currently (as of #98), the user has to annotate all the fields, they will ever mutate with assignments:

#[var(field)]
struct S {
  field: i32,
}
fn main() {
  let mut s = S { field: 123 };
  // otherwise, this fails in Stainless:
  s.field = 456;
}

This is weird in Rust, because there, the data structure is agnostic to whether it will be mutated or not. Only the place where it is stored decides about mutability. Therefore, we'd like to infer which fields are mutated in a program and automatically add the @var flag to those – relying on Rust's borrow checker to guard against forbidden mutation.

[yannbolliger]

Or have everything as @var because logically, that is correct in Rust? Might break verification.