Migrating to C++20

[marty1885]

(When C++20 finally get released and compilers got support of it) I think we should move to C++20, making a lot of stuff easier:

• No more "blablabal" + std::to_string(foo) + "blablabla" + std::to_string(bar)
  • string formatting solves the problem
• Contracts, making the code faster
• etc...

[marty1885]

We could also replace the hacky source code JIT in OpenCLBackend with std::format this way.

[marty1885]

The std::source_location feature could also improve our error message (and et_error doesn't need to be a macro)