Introducing a new directory structure in `src`. #342

Job	Run time
build (ubuntu-latest, Release, clang, 3.10)	4m 27s
build (ubuntu-latest, Release, clang, 3.11)	4m 58s
build (ubuntu-latest, Release, clang, 3.12)	5m 11s
build (windows-2019, Release, cl, 3.10)	5m 2s
build (windows-2019, Release, cl, 3.11)	4m 52s
build (windows-2019, Release, cl, 3.12)	4m 59s
build (macos-12, Release, clang, 3.10)	2m 58s
build (macos-12, Release, clang, 3.11)	3m 24s
build (macos-12, Release, clang, 3.12)	2m 58s
build (macos-13, Release, clang, 3.10)	3m 4s
build (macos-13, Release, clang, 3.11)	2m 50s
build (macos-13, Release, clang, 3.12)	3m 25s
build (macos-14, Release, clang, 3.10)	1m 31s
build (macos-14, Release, clang, 3.11)	1m 38s
build (macos-14, Release, clang, 3.12)	1m 35s
	52m 52s

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