Fix MoonBit deprecations and update Iterator APIs

[peter-jerry-ye]

Summary

• Resolved deprecation warnings across the IR package by updating APIs and refactoring affected code paths
• Migrated Iter/Iter2 usages to Iterator/Iterator2, including new iterator2 wrappers where required
• Added target and _build to .gitignore to prepare for migration tooling

Why

• The task is to eliminate as many deprecated warnings/errors as possible, including the breaking change from Iter/Iter2 to Iterator/Iterator2
• The .gitignore update is required to support future build and migration workflows

Notable Details

• Introduced new iterator2 functions that return Iterator2, while keeping legacy iter2 as a thin wrapper for compatibility
• Updated iterator construction to use the new Iterator::new/Iterator2::new callback style
• Touched multiple IR modules to align types, signatures, and iterator usage with the new APIs