Add reset to macro context

As discussed in #529 (closed) we need a way to reset the macro context. A .reset is the most obvious:

syntax m = ctx => {
  ctx.next('expr'); // foo + bar
  // oops, just wanted one token
  ctx.reset();
  let {value} = ctx.next(); // foo
  //...
}

m foo + bar

Other possible names:

• .clear
• .revert
• .rewind