feat(compiler): Always generate last argument of fold [LNG-265]

[InversionSpaces]

Description

Always generate last argument of fold instruction in result AIR to make this explicit.

Proposed Changes

Removal of Option for for and fold models

Implementation Details

Self-descriptive

Checklist

• Corresponding issue has been created and linked in PR title.
• Proposed changes are covered by tests. No tests applicable
• Documentation has been updated to reflect the changes (if applicable).
• I have self-reviewed my code and ensured its quality.
• I have added/updated necessary comments to aid understanding.

Reviewer Checklist

• Tests have been reviewed and are sufficient to validate the changes.
• Documentation has been reviewed and is up to date.
• Any questions or concerns have been addressed.

[linear]

LNG-265 Always generate `last` argument of `fold`

AquaVM implicitly deduces last instruction in fold based on the instruction argument. To make it explicit, aqua should generate:

• last = null for for and for … try
• last = never for for … par and parseq