rust-config: show default `null` is coerced incorrectly to `false` Probably being too trigger happy about boolean values, I incorrectly set the transform for a `None` to a `Some(false)`. It would cause for example the `ui.formatted` value to be set to `Some(false)`, which turns off the colors among other things, when `None` would trigger the automatic behavior. This is fixed in the next commit.

rust-config: fix fallback to default not parsing the default value When a config item's default is a string, it sometimes needs to be parsed into another type, like in the case of `cmdserver.max-log-size`, that returns a number of bytes from a human-readable amount like `25MB`. The logic for the fix is explained inline.

rust-config: demonstrate a bug when falling back to non-trivial default values The default value (when the user hasn't configured anything) is not run through the value parser, causing a programming error to happen because of type mismatch. This will be fixed in the next commit.