Support `let` without initialization

Hax doesn't support the following:

fn main() {
    let (x, y);
    x = 0;
    y = 0;
}

Draft plan to support let <PAT>; BODY:

• translate the pattern PAT into our AST, extract bindings
• for each binding, generate a let binding wrapped with in an option type
• replace any occurrence of each binder b in BODY with b.unwrap() (or make everything a monad, but that's quite heavy)