Implement arena-based heap allocation and garbage collection

[anikser]

Summary

• Replaced unsafe manual memory management with safe arena-based heap allocation
• Implemented mark & sweep garbage collection with tri-color marking
• Fixed DST implementation issues by using Vec<u8> for string storage

Changes

New Heap Module (src/common/heap.rs)

• Arena allocator: Objects stored in a Vec<Option<GcObject>> with index-based references
• Free list: Reuses freed object slots for better memory efficiency
• GC implementation: Mark & sweep with white/gray/black tri-color marking
• Root management: Explicit root set tracking for accurate collection

Updated Value Representation

• Changed Value::Object from raw pointer to ObjectRef (index)
• Removed unsafe Obj struct with manual linked list
• Fixed Box construction issue with DST types

VM Integration

• Passes heap reference to compiler for string allocation
• Manages GC roots for stack values during execution
• Updated string concatenation to use heap allocation
• Proper object display handling through heap lookups

Benefits

• Memory safety: No more raw pointers or manual deallocation
• Automatic GC: Configurable threshold-based collection
• Better performance: Arena allocation reduces fragmentation
• Rust idiomatic: Uses safe abstractions throughout

Test plan

• Basic string allocation works
• String concatenation allocates properly
• GC can be triggered manually
• Add comprehensive GC stress tests
• Benchmark GC performance

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