Performance bottlenecks

[adragnevVW]

Describe the bug
Performance bottlenecks exit in GetSymDefAt() and GetSymDefCount(). Several collection access methods use inefficient searches.

To Reproduce
Steps to reproduce the behavior:

1. Load a large MVR file with 1000+ objects of mixed types (symbols).
2. Call GetSymDefAt() in a loop.
3. Observe significant performance degradation as collection size increases.

Expected behavior
• Large datasets should be handled efficiently without performance degradation.