Work towards elixir 14

While here, make the object be an elixir struct instead of delegating to
the erlang code.

As part of this, geef_nif:commit_tree() has been fixed to return the id,
insted of the type, as we know it's going to be a tree, but not then id.

lib/geef/pkt.ex

@@ -1,6 +1,8 @@
 require Record
 
-defrecord Geef.Request, Record.extract(:geef_request, from: "src/geef_records.hrl")
+defmodule Geef.Request do
+  defstruct Record.extract(:geef_request, from: "src/geef_records.hrl")
+end
 
 defmodule Geef.Pkt do