Speed up build

[tomjnixon]

libadm is quite slow to build, which makes it less pleasant to work on than it could be. This issue is here to collect ideas and track progress.

Some ideas:

• build all the tests into one executable, and use catch cmake integration to make it nice to work with (some progress)
• speed up compilation of individual objects, mostly by reducing the amount of code pulled in by headers
  • set up IWYU and follow it's recommendations (some progress)
  • where possible, use headers containing forward declarations of boost::optional and similar
  • improve the use of forward declarations in the library
    • currently elements_fwd.hpp isn't that useful, because headers for elements depend on ParameterTraits<T>::tag, which only works if the real header for T is included. This means that most of them end up including full headers for all other types. This can be improved by adding ParameterTraits overloads for all elements in type_traits.hpp or elements_fwd.hpp.
    • document.hpp should use elements_fwd.hpp, but document.hpp is used as a root header to include everything. It should probably be turned into a wrapper that includes elements.hpp and document_impl.hpp, and internal parts which need a definition of Document should be changed to include document_impl.hpp
  • consider moving implementations out of common headers like id_assignment.hpp and into implementation files. In this case the header should only need the forward declaration for IDs, not all the full declarations!

[rsjbailey]

Try profiling build time using (for example) https://devblogs.microsoft.com/cppblog/analyze-your-builds-programmatically-with-the-c-build-insights-sdk/ to identify what is slow.

Looks like clang has https://aras-p.info/blog/2019/01/16/time-trace-timeline-flame-chart-profiler-for-Clang/