Modulizating Etaler (C++20 modules)

[marty1885]

It's too early to start doing this. Putting a note here so I remember it later on.

Etaler has a pretty bad build time for it's code size. Mostly due to the extensive use of complex STL algorithms, TMP and OpenCL C++ wrapper template symbols. C++20's modules should be able to solve this.

But for starter (before we fully transition to C++20) let's make a module using the existing headers and see how it goes.