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.