generators and asyncGenerators should be immutable

[richytong]

generators and generated iterators should be immutable. right now they are not; methods like fork deplete the same shared iterator.

const generateIncrements = n => function*(s) {
  for (let i = 0; i < s.length; i += n) {
    yield s.slice(i, i + n)
  }
}

fork([
  reduce(add),
])(generateIncrements(2)('aaaaaaaaaa')) // => ['aaaaaaaaaa']

fork([
  reduce(add),
  reduce(add),
])(generateIncrements(2)('aaaaaaaaaa')) // TypeError: reduce(...)(x); x cannot be empty

We should have seen ['aaaaaaaaaa', 'aaaaaaaaaa']

I believe we can do this by accessing a generator's Symbol.iterator or Symbol.asyncIterator

[richytong]

At the moment I'm against enforcing immutability in this library