Add `diagonal` as a generalization of `LinearAlgebra.Diagonal` #18

Job	Run time
Tests (1, macOS-latest) / Julia 1 - macOS-latest	1m 52s
Tests (1, ubuntu-latest) / Julia 1 - ubuntu-latest	2m 31s
Tests (1, windows-latest) / Julia 1 - windows-latest	3m 15s
Tests (lts, macOS-latest) / Julia lts - macOS-latest	3m 24s
Tests (lts, ubuntu-latest) / Julia lts - ubuntu-latest	3m 29s
Tests (lts, windows-latest) / Julia lts - windows-latest	4m 23s
	18m 54s

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