Merge remote-tracking branch 'origin/mr/491'

  Major changes are described below.  We expect that we're mostly

  done with breaking changes in this area, though we expect a few

  minor API breaks here in the next release.

  - Lists of objects, and contained configuration objects objects, are

  - Lists of objects, and contained configuration objects, are

    now constructed using a uniform pattern.

  - All of our config _builder_ types are now `Deserialize`; our

    configuration types themselves are not.

### Cleanups, minor features, and minor bugfixes

- We report more accurate and useful messages on failure to build a

  circuits. ([f7810d42eb953bf5])

  circuit. ([f7810d42eb953bf5])

- Avoid dropping information when reloading guards. ([#429])

- Arti now treats expired or not-yet-valid directory objects as an error

  condition, since they indicate that the directory cache (or the