Defer BroadcastStyle construction to back-ends. #559

Job	Run time
Documentation	17s
Julia 1.8 - ubuntu-latest - x64	13m 6s
Julia 1.8 - macOS-latest - x64	20m 54s
Julia 1.8 - windows-latest - x64	14m 2s
Julia 1.9 - ubuntu-latest - x64	12m 26s
Julia 1.9 - macOS-latest - x64	23m 8s
Julia 1.9 - windows-latest - x64	13m 30s
Julia 1.10.0-beta2 - ubuntu-latest - x64	12m 19s
Julia 1.10.0-beta2 - macOS-latest - x64	19m 11s
Julia 1.10.0-beta2 - windows-latest - x64	14m 17s
Julia nightly - ubuntu-latest - x64	17m 37s
Julia nightly - macOS-latest - x64	23m 11s
Julia nightly - windows-latest - x64	18m 45s
	3h 22m 43s

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