[Refactor] Change standalone export structure to better handle multi backends

Context

It is now possible to export a model using several export backends.
It allows for a new export to use the export_cpp kernels, alongside with its own optimized kernels.
However these files need to coexist in the generated standalone export.

This MR brings a standard to ensure compatibility between the backends.
The folder tree of the generated export should look like this :

kernels/  
   cpp/  
      cpp_kernel.hpp  
   my_export/  
      my_export_kernel.hpp  
utils/  
   cpp/  
      cpp_utils.hpp  
   my_export/  
      my_export_utils.hpp  

A system of namespace has been added too, to avoid errors at compile time.

Changes

Refactor

• New folder tree;
• Namespaces for static files;