Minor features, many bug fixes

- Fix unsafe pointer arithmetic panics with -race enabled.
- Fix Close panic if ctx is canceled during Open.
- Update to SQLite3 3.32.0
- Add Conn.ChangesetApplyInverse

Thanks to Ross Light (@zombiezen):
- Add Conn.SetAuthorizer.
- Add Conn.Limit method.

Thanks to Vincent Rischmann (@vrischmann):
- Fix `go mod vender` not including C files.