Defer BroadcastStyle construction to back-ends. #557

Job	Run time
Documentation	12s
Julia 1.8 - ubuntu-latest - x64	14m 17s
Julia 1.8 - macOS-latest - x64	21m 45s
Julia 1.8 - windows-latest - x64	29s
Julia 1.9 - ubuntu-latest - x64	12m 23s
Julia 1.9 - macOS-latest - x64	18m 36s
Julia 1.9 - windows-latest - x64	30s
Julia 1.10.0-beta2 - ubuntu-latest - x64	12m 40s
Julia 1.10.0-beta2 - macOS-latest - x64	18m 0s
Julia 1.10.0-beta2 - windows-latest - x64	31s
Julia nightly - ubuntu-latest - x64	17m 0s
Julia nightly - macOS-latest - x64	22m 43s
Julia nightly - windows-latest - x64	44s
	2h 19m 50s

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