Faster compile times

Right now it takes very long on my laptop to compile hops with tests (the library itself is not that slow).

Here are the measured times:

debug build make -j2
real 7m48.094s
user 14m46.499s
sys 0m36.942s


release build make -j2
real 16m12.366s
user 31m28.923s
sys 0m22.815s

Since we adopted boost test after using google test, the way tests are managed in the CMakeLists.txt is not optimal. If we were to adopt the structure to something more like this https://www.jetbrains.com/help/clion/boost-test-support.html, then I would expect the compilation to speed up significantly. The only question is, whether compiling a single test would still be fast. We should test this and if single test compile times suffer too much, we can think about adding precompiled hops-headers.