Capture with closures

A closure writes |parameters| expression or returns a block. Types may be inferred from context. Captured bindings are shared by reference, so mutations inside and outside observe the same value.

A let-bound closure may call itself. Give recursive closures an explicit return type when the base case cannot make it obvious.

fn main() {
  let mut count = 4
  let bump = || -> int { count += 1; count }
  println(bump())
  println(bump())
  println(count)
}