Iterator API with data copy

[efirs]

Implement HasNext/Get APIs, which returns an object suitable to be passed outside of iteration loop scope.

Example API:

for it.HasNext() {
   d := it.Get()
}

it.Get() would return a pointer to the document.

Current API we have, minimizes allocations and copies.
But the object is only accessible inside the loop scope.

var d Doc
for it.Next(&doc) {
}

[ankitsridhar16]

I would like to work on this