Remove unused Arena implementation, consolidate to CompactArena only

CLEANUP ANALYSIS:
✅ Found that only CompactArena is actually used in BPlusTreeMap
✅ Generic Arena was dead code - only used in tests and examples
✅ Eliminated code duplication and confusion

CHANGES:
- Removed arena.rs (519 lines of unused code)
- Moved all arena allocation methods to compact_arena.rs
- Updated lib.rs imports to remove Arena references
- Consolidated all arena operations in one place

ARENA METHODS MOVED TO compact_arena.rs:
✅ allocate_leaf() & allocate_branch()
✅ deallocate_leaf() & deallocate_branch()
✅ free_leaf_count() & allocated_leaf_count()
✅ free_branch_count() & allocated_branch_count()
✅ leaf_utilization() & branch_utilization()
✅ leaf_arena_stats() & branch_arena_stats()
✅ set_leaf_next()
✅ get_leaf_unchecked() & get_branch_unchecked()

RESULTS:
✅ compact_arena.rs: 360 → 461 lines (+101 lines of arena management)
✅ lib.rs: 285 lines (unchanged - no duplication)
✅ Removed 519 lines of dead code (arena.rs)
✅ All 317 tests pass - 100% functionality preserved
✅ Clean compilation with no errors
✅ Single source of truth for arena operations

BENEFITS:
- Eliminated dead code and confusion
- Single arena implementation (CompactArena)
- Better performance (no Option wrapper overhead)
- Cleaner codebase with focused responsibilities
- Easier maintenance and understanding

Author: KentBeck