Update derived categories via expression

[gergness]

exploring what it would look like to use expressions to update categories for derived variables instead of the endpoint.

Would need more zcl hacking than I'd like until https://www.pivotaltracker.com/story/show/174173987, so am pausing work for now.

[lintr-bot]

R/categories.R:272:1: style: Lines should not be more than 100 characters.

​            !isTRUE(all.equal(old[[attr]], new[[attr]])) && !(is.na(old[[attr]]) && is.null(new[[attr]])) && !(is.null(old[[attr]]) && is.na(new[[attr]]))
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

R/categories.R:278:1: style: Lines should not be more than 100 characters.

​    # We'll use `selectCategories()` for selection changes, but `alterCategories()` for everything else
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

R/expressions.R:617:1: style: Lines should not be more than 100 characters.

​    if (!is.null(category_order)) args$order <- list(value = alter_cats_get_order_ids(x, category_order))
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~