* updated for data spec 0.4.0
* added two iterators
- iter_full, including iteration with context
- iter_struct
* renamed SourceDocument to SrcDocument
* local load/dump split from SrcDocument, into a LocalSrcDocument class +
a module function
* added base classes for table, tree & sequence docs
* chunk payload change from `text` to `data`, to make it more generic
* added format indicator strings to output dump