next-hardfork-changes.md

Next hardfork changes:

• computing subtrace lengths in FoldFSM (for more details see PR 138)
• change Sender struct serialization way in CallResult::RequestSentBy
• add a separate (empty?) state in air_interpreter_data::CallResult for CallOutputValue::None for hardening
• remove serde-based field renaming in data to support outdated data versions