Closes #80 and #85 Speed up derive functions and remove `by_vars` #416

Job	Run time
Get R version	1s
Version Bump	0s
Links / Validate Links	5s
Check / (release)	4m 30s
Check / (devel)	44s
Check / (oldrel)	4m 52s
Documentation	0s
Validation	0s
Lint / Lint	2m 17s
Code Coverage / Test Coverage	2m 57s
Spelling / Spellcheck	2m 14s
Man Pages / Roxygen	2m 54s
Code Style / Code Style	2m 22s
Check Templates / Verify	2m 44s
Code Coverage / Generate badge for coverage	6s
	25m 46s

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