Use IndexMap in byte_seq_parser macro

[osa1]

This make map iteration deterministic and makes tiny builds reproducible.

Fixes #437.

[osa1]

The command

  cargo test --no-default-features \
    --features "tls-native desktop-notifications"

Is currently failing on macos-latest. It started a while ago and the run passes if I restart it a few times. We should investigate it separately.

[bmwiedemann]

Thanks for the quick patch. I cannot test it easily with my tools because of the added dependency. I expect it to be fine.