IArray: avoid heap allocation if there is only one element #71

Job	Run time
test (ubuntu-latest, stable, 1.9.3)	34s
test (ubuntu-latest, stable, latest)	35s
test (ubuntu-latest, 1.64.0, 1.9.3)	4m 33s
test (ubuntu-latest, 1.64.0, latest)	4m 37s
test (windows-latest, stable, 1.9.3)	1m 38s
test (windows-latest, stable, latest)	1m 51s
test (windows-latest, 1.64.0, 1.9.3)	7m 23s
test (windows-latest, 1.64.0, latest)	5m 59s
test (macos-latest, stable, 1.9.3)	53s
test (macos-latest, stable, latest)	34s
test (macos-latest, 1.64.0, 1.9.3)	6m 12s
test (macos-latest, 1.64.0, latest)	5m 2s
	39m 51s

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