It uses CMake 3.16 built-in utilities to speed up build time:
* Unity builds
* Precompiled headers
The feature is controlled via `ENABLE_FASTER_BUILD` CMake option (disabled by default).
The option avaialble only on CMake >= 3.16.
The feature is enabled per-target via `ie_faster_build` function.
Some observations:
* Don't have actual numbers for compile time, but subjectively can see
speed up locally with VS 2019.
* Unity builds gives much more effect, but has some restriction on source files,
so are not used everywhere.
