FileSystem.copyItem can parallelise directory copy #228

Job	Run time
Soundness / API breakage check	2m 29s
Soundness / Format check	50s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	2m 2s
Soundness / License headers check	8s
Soundness / Unacceptable language check	5s
Swift 6 Language Mode / Swift 6 language mode	1m 1s
Integration Tests / Linux (5.8)	12m 59s
Soundness / Shell check	7s
Integration Tests / Linux (5.9)	14m 36s
Integration Tests / Linux (5.10)	15m 5s
Integration Tests / Linux (nightly-6.0)	17m 6s
Integration Tests / Linux (nightly-main)	17m 39s
Cxx interop / Cxx interop / Linux (5.8)	57s
Cxx interop / Cxx interop / Linux (5.9)	1m 5s
Cxx interop / Cxx interop / Linux (5.10)	1m 3s
Cxx interop / Cxx interop / Linux (nightly-6.0)	1m 46s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 44s
Benchmarks / Benchmarks / Linux (5.8)	2m 31s
Benchmarks / Benchmarks / Linux (5.9)	3m 44s
Benchmarks / Benchmarks / Linux (5.10)	3m 32s
Benchmarks / Benchmarks / Linux (nightly-6.0)	5m 17s
Benchmarks / Benchmarks / Linux (nightly-main)	5m 27s
Unit tests / Unit tests / Linux (5.8)	3m 17s
Unit tests / Unit tests / Linux (5.9)	3m 28s
Unit tests / Unit tests / Linux (5.10)	3m 18s
Unit tests / Unit tests / Linux (nightly-6.0)	4m 18s
Unit tests / Unit tests / Linux (nightly-main)	4m 20s
	2h 9m 57s

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