Make the public API a bit tidier.

- Export only public API.
- Rename: 'get_or_create_table' --> 'get_table'
- Rename: 'create_table'        --> 'declare_table'
- Mapper function should accept a single (key, value) tuple rather than
  a key and a value as separate params.
- Tidier comments.

Change-Id: I6814232da062563c236aeb4e26ae4fa1378cf22e