wscript: functions and closures

fn definitions, function values as parameters, and closures that capture by reference.

fn area(w: int, h: int) -> int { w * h }
fn apply(f: fn(int) -> int, x: int) -> int { f(x) }
let double = |x| x * 2          // closures capture by reference

Functions are first-class: a fn(int) -> int type annotates a function parameter, and closures (|x| …) capture their environment by reference.