Implement 'with' statement

[ramonasuncion]

• A new 'with' statement provides syntactic sugar for monadic control flow.
  Within a block, the subsequent statements after a 'with' statement are
  passed as a capture-by-value lambda to the function specified as the 'with'
  statement's operand.
  // 0.2
  foo(x) {
  with y = bar(x);
  with z = bas(y);
  qux(z);
  }

  // desugars to

  foo(x) {
  bar(y => {
  bas(z => {
  qux(z);
  }, y);
  }, x);
  }
  Thanks to Arvid Picciani.