This commit collects all the time-related fields from the Environment and bundles them all together in their own encapsulated struct.

This commit extracts the common table element from the details and grid_details modules, and makes it its own reusable thing.

- A Table no longer holds the values it’s rendering; it just holds a continually-updated version of the maximum widths for each column. This means that all of the resulting values that turn into Rows — which here are either files, or file eggs — need to be stored *somewhere*, and that somewhere is a secondary vector that gets passed around and modified alongside the Table.
- Likewise, all the mutable methods that were on Table that added a Row now *return* the row that would have been added, hoping that the row does get stored somewhere. (It does, don’t worry.)
- Because rendering with mock users is tested in the user-field-rendering module, we don’t need to bother threading different types of U through the Environment, so now it’s just been specialised to UsersCache.
- Accidentally speed up printing a table by not buffering its entire output first when not necessary.