refactored gemm-related code. added gemm-related optimizations and tests. added specializations for mul

added a main method for test. That way we don't need to change code in each test_file.py fi we want to change what it do if we call it directly.

-found bug: super().initialize don't work
-default_initialization don't overwrite the value. This is unexpected comportement.

added a default_initialize method that is missing when we use the _instance_initialize method in a module.

added allow_gc option to PerformLinker, which frees intermediate results during execution when they are no longer needed.