This API isn't fantastic, but it's the best I can come up with without
something like `concat_idents!()`. There are relatively few places where
config is set, to hopefully the ugliness isn't disastrous.
Change previous occurences of `config.item = value` to this new API,
rather than using `config.override_value()`. Undo the changes to
`override_value()`, as it's no longer important to propogate the error
to the caller. Add a test for the new interface.
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