Refactor the code into separate files

Separate repository/odb functions into their own file. Add odb_exists
to go through the repository's odb. Adjust the test script.

c_src/geef.h

@@ -0,0 +1,18 @@
+#ifndef GEEF_H
+#define GEEF_H
+
+#include "erl_nif.h"
+
+ERL_NIF_TERM geef_error(ErlNifEnv *env);
+
+typedef struct {
+    ERL_NIF_TERM ok;
+    ERL_NIF_TERM error;
+    ERL_NIF_TERM true;
+    ERL_NIF_TERM false;
+    ERL_NIF_TERM repository;
+} geef_atoms_t;
+
+extern geef_atoms_t geef_atoms;
+
+#endif