Document. add get(i) and addAll to make interacting with fieldables and documents easier/faster and more readable [LUCENE-2540]

Working with Document Fieldables is often a pain.
getting the ith involves chained method calls and is not very readable:

```Java
// nice
doc.getFieldable(i);

// not nice
doc.getFields().get(i);
```

also, when combining documents, or otherwise aggregating multiple fields into a single document,

```Java
// nice
doc.addAll(fieldables);

// note nice: less readable and more error prone
List fields = ...;
for (Fieldable field : fields) {
result.add(field);
}
```

---
Migrated from [LUCENE-2540](https://issues.apache.org/jira/browse/LUCENE-2540) by Woody Anderson, updated May 09 2016
Attachments: [LUCENE-2540.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2540/LUCENE-2540.patch)

Start at the Document API and its Fieldable access shown in the issue, then review the linked LUCENE-2540.patch for the intended scope. Done means Document supports the requested get(i) and addAll interactions described in the examples, with relevant tests passing.

Written by the indexing model from the issue text.