Add construction warning

Fix indentation and broken links

Re-think planned features

Update README

Remove `set.seed()` from individual variable simulations. Setting the seed each time results in identical sampling

Fix typo

Fix typo

Improve `print()` method

Create README

Add {tibble}, {tidyselect}

Implement `simulate()`, `assert()`, and a `print()` method for dataset promises

Add utilities

Implement `declare_survey_logic()` and two generators `skipped()`, `imputed()`

Implement `declare_derived_variables()`

Set `missing_perc = NA` by default, meaning we don't check missingness at all in validators