Provide and use macro for impl Derive via Builder

Ian Jacksonrequested to merge
Diziet/arti:builder-default into main

  1. There were a lot of open-coded impl Default ... FooBuilder::default().build()....

  2. There were three cases where we #[derive(Default)]. As it happens none of these had non-Default default fields, so there wasn't an actual bug.

Edited by Ian Jackson

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