Add export_manifest function #132

Job	Run time
R Package Checks / docs-check / docs-check	1m 34s
R Package Checks / codecov / test-coverage	2m 4s
R Package Checks / lintr / lint	1m 17s
R Package Checks / check-R-updates / updated-files	6s
R Package Checks / r-cmd-check / macOS-latest (release)	3m 21s
R Package Checks / r-cmd-check / windows-latest (release)	7m 44s
R Package Checks / r-cmd-check / windows-latest (3.6)	3m 50s
R Package Checks / r-cmd-check / windows-latest (4.1)	5m 46s
R Package Checks / r-cmd-check / ubuntu-latest (release)	2m 3s
R Package Checks / r-cmd-check / ubuntu-latest (devel)	3m 39s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-1)	1m 57s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-2)	2m 1s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-3)	2m 11s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-4)	1m 51s
R Package Checks / version-check / version-check	1m 5s
	40m 29s

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