Defer BroadcastStyle construction to back-ends. #561

Job	Run time
Documentation	12s
Julia 1.8 - ubuntu-latest - x64	13m 11s
Julia 1.8 - macOS-latest - x64	23m 48s
Julia 1.8 - windows-latest - x64	13m 52s
Julia 1.9 - ubuntu-latest - x64	12m 51s
Julia 1.9 - macOS-latest - x64	18m 3s
Julia 1.9 - windows-latest - x64	13m 49s
Julia 1.10.0-rc2 - ubuntu-latest - x64	12m 30s
Julia 1.10.0-rc2 - macOS-latest - x64	15m 36s
Julia 1.10.0-rc2 - windows-latest - x64	13m 51s
Julia nightly - ubuntu-latest - x64	17m 17s
Julia nightly - macOS-latest - x64	27m 54s
Julia nightly - windows-latest - x64	18m 31s
	3h 21m 25s

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