compare_by plotter: filenames too long for large symbols

[JPenuchot]

The compare_by plotter saves plots with filenames generated using the key itself. This becomes problematic when the symbols grow large enough for filenames to be too long.

The issue is known and currently has no other solution than generating smaller symbols at the moment, eg. benchmark driver functions with names that do not depend on benchmark instantiation size.

Solutions for this problem are welcome. I'm open to having different name shortening strategies, generating index files to help find plot files, and others. I will address this issue if it becomes really problematic.