Closes #80 and #85 Speed up derive functions and remove `by_vars` #424

Job	Run time
Get R version	1s
Version Bump	0s
Links / Validate Links	4s
Check / (release)	4m 33s
Check / (devel)	55s
Check / (oldrel)	4m 49s
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Validation	0s
Code Coverage / Test Coverage	2m 45s
Lint / Lint	2m 20s
Man Pages / Roxygen	2m 37s
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Code Coverage / Generate badge for coverage	9s
	25m 52s

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