Remove, replace, or better test&document `quiet`, `enlist`, `list2var`

Either:
- Remove `quiet`, and find an alternative for its use in growth_rate.R (`capture.output`? `suppressMessages`? something more selective? `sink(nullfile())`? ...)
- Document that `quiet` leaves around tempfiles with the captured output

Either:
- Remove `enlist` and its test, as it appears to be unused.
- Replace `enlist` with `tibble::lst`, as it appears to serve a similar purpose, but is more battle-tested, separately maintained, and more featureful.
- Augment the `enlist` testthat test to test its special behaviors vs. just `list`; e.g., `enlist(2+2)`.

Remove `list2var`, which is unused and broken.

Start by locating quiet, enlist, list2var, the growth_rate.R call site, and the existing testthat test for enlist. Review their usages and coverage before choosing the accepted path for each helper. Done means the unused or broken functionality is addressed, quiet's tempfile behavior is documented or replaced, and retained behavior has focused tests.

Written by the indexing model from the issue text.