Add imbl immutable collections support

[tobiemh]

Summary

This PR adds support for serializing and deserializing imbl immutable data structures, with optimized deserialization using bulk construction patterns.

Changes

• Add new optional imbl feature and dependency
• Implement Revisioned, SerializeRevisioned, and DeserializeRevisioned traits for:
  • imbl::Vector<T>
  • imbl::OrdMap<K, V>
  • imbl::OrdSet<T>
  • imbl::HashMap<K, V>
  • imbl::HashSet<T>
• Add comprehensive benchmarks comparing imbl::OrdMap vs std::collections::BTreeMap for building, cloning, clone+modify, serialization, and deserialization

Implementation Details

The deserialization implementation uses an optimized pattern that:

1. Pre-allocates a Vec with the known capacity for better cache locality during reads
2. Uses compiler hints (std::hint::assert_unchecked) to optimize bounds checking
3. Converts to the imbl type via FromIterator for efficient bulk construction

This approach is significantly faster than inserting elements one-by-one into the persistent data structures.

Testing

• Unit tests for all supported types (including empty collection edge cases)
• Performance benchmarks demonstrate the serialization/deserialization characteristics