Make foldMap stack safe

Running this snippet results in `RangeError: Maximum call stack size exceeded`:

``` js
const Identity = require('fantasy-identities')
const Free = require('@safareli/free')
const runTimes = (n) => (v) => {
  console.log(v);
  const res = Free.liftF(n)
  if (n == 0){
    return res
  }
  return res.chain(runTimes(n-1))
}
console.log(runTimes(1000000)().foldMap(Identity,Identity))
```
