Add (de-)serialization support for Iterable

[Marcono1234]

This adds only support for (de-)serializing Iterable, but not any of its subtypes since it would not be possible to properly deserialize them again (see also #1708 and #1389).

The targeted use case is classes with Iterable fields:

class CustomIterable extends Iterable<Integer> {
  public Iterator<Integer> iterator() {
    ...
  }
}

class MyClass {
  Iterable<Integer> iterable;
}

public static void main(String... args) {
  MyClass obj = new MyClass();
  obj.iterable = new CustomIterable();
  gson.toJson(obj); // = {"iterable":[...]}
}

However, since it only supports Iterable (and no subtypes) it might not be useful enough. It would be good to gather some feedback on how useful it would be and which use cases it would suit.

Note that these changes might have an effect for serialization of fields with compile-time type Iterable for which previously the reflective adapter for the runtime type was used (see how TypeAdapterRuntimeTypeWrapper chooses the type adapter).