Open details to the public API

[vinniefalco]

In order that authors be able to write tools for nudb, details such as the file format types (e.g. detail::dat_file_header) and the functions to interact with it need to be part of the public interface.

[vinniefalco]

Although, the rekey operation should probably be a public function instead of exposing ALL the details such as the bucket structures.

[vinniefalco]

rekey is a public API now, and I'm having second thoughts about exposing all the details. Because then people might rely on it. There should be an understanding, that third party tools which rely on the implementation details of the database, might break when new versions are released. So its okay to leave things in detail namespace.

I'm open to counter arguments