bpo-42064: Optimise sqlite3 state access, part 1 (GH-27273)

Prepare for module state:

- Add "get state by defining class" and "get state by module def" stubs
- Add AC defining class when needed
- Add state pointer to connection context
- Pass state as argument to utility functions

Automerge-Triggered-By: GH:encukou

Modules/_sqlite/util.h

@@ -35,7 +35,7 @@ int pysqlite_step(sqlite3_stmt *statement);
  * Checks the SQLite error code and sets the appropriate DB-API exception.
  * Returns the error code (0 means no error occurred).
  */
-int _pysqlite_seterror(sqlite3 *db);
+int _pysqlite_seterror(pysqlite_state *state, sqlite3 *db);
 
 sqlite_int64 _pysqlite_long_as_int64(PyObject * value);