Create better ways to get a list of posts/all posts, using :ets.select and match specs

[alfredbaudisch]

I intended to do this since the beginning, being one of the reasons that I picked up ETS for the project. So documenting it here.

For the ones new about the subject, some starting points:

• https://erlang.org/doc/apps/erts/match_spec.html
• https://elixirforum.com/t/best-way-to-get-multiple-keys-from-static-ets-table/23692
• https://stackoverflow.com/questions/64188452/erlang-etsselect-sublist
• https://learnyousomeerlang.com/ets