IPackable/IUnpackable should be respected even in collections

[yfakariya]

Derived from #150.
If the collection (that means types implementing IEnumerable) implements IPackable and/or IUnpackable, the generated serializer should respect them.
To keep backward compatibility, SerializationContext should have compatibility switch which turn of this new (fixed) bahavior.

[yfakariya]

This issue was fixed in 0.7.0-alpha3, so I've closed it.