Use OrdinalIgnoreCase instead of InvariantCultureIgnoreCase

[smedilol]

When comparing String, OrdinalIgnoreCase is supposed to be faster than InvariantCultureIgnoreCase.

As the Cobol source code is not Culture dependant, we can switch to OrdinalIgnoreCase in SymbolTable and compare performance.

[cobarzanc]

Indeed, the OrdinalIgnoreCase StringComparer is much faster than InvariantCultureIgnoreCase. A performance comparison is available here (https://blogs.msdn.microsoft.com/noahc/2007/06/29/string-equals-performance-comparison/). Using OrdinalIgnoreCase brings arround 5x improvement in speed when compared to InvariantCultureIgnoreCase (in .Net 3.5).

Microsoft take on best practices for string manipulation - https://docs.microsoft.com/en-us/dotnet/standard/base-types/best-practices-strings