Add export_manifest function #117

Job	Run time
R Package Checks / codecov / test-coverage	1m 48s
R Package Checks / docs-check / docs-check	1m 34s
R Package Checks / lintr / lint	1m 28s
R Package Checks / check-R-updates / updated-files	5s
R Package Checks / r-cmd-check / macOS-latest (release)	3m 28s
R Package Checks / r-cmd-check / windows-latest (release)	7m 13s
R Package Checks / r-cmd-check / windows-latest (3.6)	5m 0s
R Package Checks / r-cmd-check / windows-latest (4.1)	5m 44s
R Package Checks / r-cmd-check / ubuntu-latest (release)	2m 2s
R Package Checks / r-cmd-check / ubuntu-latest (devel)	3m 15s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-1)	2m 32s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-2)	2m 12s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-3)	3m 13s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-4)	2m 9s
R Package Checks / version-check / version-check	1m 23s
	43m 6s

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