Use impl_standard_builder more and remove manual Default/builder impls

The first commit here is the same as the first commit of !589 (merged). Like that MR, this branch introduces new tests, one of which spots that bug. That MR introduces what might be seen as an integration test.

When working on !589 (merged) it occurred to me to wonder why the existing deser unit tests hadn't caught this. Well, the answer is, they're in impl_standard_builder, which wasn't being used.

To spot what to put in this branch I did git-grep '.*pub.*fn.*builder' and set about replacing those that seemed like parts of the config. (I also did a git-grep 'impl Default', resulting in one commit, but that had too many false positives to pursue.)