Implement `pl$duration()` #2269

Job	Run time
macos-latest with pre-built binary (release)	36s
macos-latest (release)	6m 11s
windows-latest with pre-built binary (oldrel-1)	31s
windows-latest (devel)	18m 36s
windows-latest with pre-built binary (release)	28s
windows-latest (release)	17m 43s
windows-latest with pre-built binary (devel)	29s
ubuntu-latest (devel)	4m 31s
ubuntu-latest with pre-built binary (oldrel-1)	14s
ubuntu-latest with pre-built binary (release)	13s
ubuntu-latest (release)	3m 55s
ubuntu-latest (oldrel-1)	4m 6s
ubuntu-latest with pre-built binary (devel)	10s
macos-latest (release) full-features	8m 36s
windows-latest (release) full-features	17m 26s
ubuntu-latest (release) full-features	4m 3s
	1h 27m 48s

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