Closes #80 and #85 Speed up derive functions and remove `by_vars` #421

Job	Run time
Get R version	1s
Version Bump	0s
Links / Validate Links	4s
Check / (release)	4m 33s
Check / (devel)	54s
Check / (oldrel)	4m 46s
Documentation	0s
Validation	0s
Spelling / Spellcheck	2m 14s
Lint / Lint	2m 21s
Code Coverage / Test Coverage	3m 6s
Man Pages / Roxygen	2m 40s
Code Style / Code Style	2m 33s
Check Templates / Verify	3m 0s
Code Coverage / Generate badge for coverage	5s
	26m 17s

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