Make vignette building faster

[nmdefries]

Make vignette building work at interactive speeds.

Most vignettes take ~10s to build, and growth_rate.Rmd and compactify.Rmd are major time hogs, taking ~1min a piece. build_vignettes() takes around 3min.

More context here.

[nmdefries]

On my computer, all vignettes build in 2 min. Slowest builds are: growth_rate.Rmd at 1 min, slide.Rmd at 21 s, compactify.Rmd at 17 s.

Profiling all the code in growth_rate.Rmd, most (~75%) of the time is spent doing actual modeling/fitting steps. So this will be hard to speed up, other than just using less data.

cv.trendfilter and trendfilter are only used in one branch of growth_rate(), so maybe we can do fewer examples using those settings.