Rename internal functions

_write_* → _dump_*
_read_* → _load_*

The code is now laid out very regular:

* package preface
* api
  * export functions
  * OO methods
* private/internal methods
  * load internals
    * hash/array/scalar
  * dump internals
    * hash/array/scalar
* helper internals
* documentation

That's it!