Allow registering boxed systems #211

Job	Run time
build (windows-latest)	6m 13s
build (ubuntu-latest)	1m 46s
check-missing-examples-in-docs	58s
check-bevy-internal-imports	3s
ci	2m 17s
build (macos-latest)	2m 17s
miri	9m 43s
run-examples-on-windows-dx12	18m 16s
check-doc	9m 42s
check-missing-features-in-docs	39s
markdownlint	1m 8s
check-compiles	2m 19s
msrv	52s
build-wasm	42s
	56m 55s

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