Add WorldManager and world save/load #148

Job	Run time
build (macos-latest, Release, clang, clang++)	3m 14s
build (ubuntu-latest, Release, clang, clang++)	2m 42s
build (macos-latest, Debug, clang, clang++)	1m 39s
build (ubuntu-latest, Debug, clang, clang++)	3m 1s
build (windows-latest, Release, clang, clang++)	2m 45s
build (windows-latest, Debug, clang, clang++)	2m 26s
	15m 47s

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