Implement `pl$duration()` #2270

Job	Run time
macos-latest with pre-built binary (release)	40s
macos-latest (release)	11m 17s
windows-latest with pre-built binary (oldrel-1)	31s
windows-latest (devel)	18m 28s
windows-latest with pre-built binary (release)	28s
windows-latest (release)	18m 30s
windows-latest with pre-built binary (devel)	31s
ubuntu-latest with pre-built binary (oldrel-1)	15s
ubuntu-latest (devel)	4m 47s
ubuntu-latest with pre-built binary (release)	13s
ubuntu-latest (release)	4m 34s
ubuntu-latest with pre-built binary (devel)	17s
ubuntu-latest (oldrel-1)	4m 26s
macos-latest (release) full-features	11m 1s
windows-latest (release) full-features	17m 53s
ubuntu-latest (release) full-features	5m 29s
	1h 39m 20s

Provide feedback