Compilation speed-up

[elshize]

• [ ] https://github.com/include-what-you-use/include-what-you-use
• [ ] create a translation unit in src that, say, compiles query processing stuff once, and exposes an easy-to-use interface from each src file
• [ ] extern templates
• [ ] forward declarations
• [ ] precompiled headers https://github.com/nanoant/CMakePCHCompiler
• [ ] ?

[elshize]

We can use -ftime-report flag to analyze.