Defer BroadcastStyle construction to back-ends. #562

Job	Run time
Julia 1.8 - ubuntu-latest - x64	13m 0s
Documentation	44s
Julia 1.8 - macOS-latest - x64	24m 42s
Julia 1.8 - windows-latest - x64	13m 59s
Julia 1.9 - ubuntu-latest - x64	12m 19s
Julia 1.9 - macOS-latest - x64	19m 38s
Julia 1.9 - windows-latest - x64	13m 43s
Julia 1.10.0-rc2 - ubuntu-latest - x64	12m 50s
Julia 1.10.0-rc2 - macOS-latest - x64	20m 2s
Julia 1.10.0-rc2 - windows-latest - x64	13m 31s
Julia nightly - ubuntu-latest - x64	17m 56s
Julia nightly - macOS-latest - x64	21m 46s
Julia nightly - windows-latest - x64	18m 19s
	3h 22m 29s

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