Non-deterministic results for overlapping overrides

Source::collect returns a config::Map which is a HashMap. HashMap traversals are unordered, so the processing order of a Source is non-deterministic.

This can be an issue if multiple values in the Source affect the same data:

    #[derive(Debug, Serialize, Deserialize)]
    struct TestConfig {
        nested: std::ops::Range<i32>,
    }


impl Source for ExampleSource {
    fn collect(&self) -> Result<config::Map<String, config::Value>, config::ConfigError> {
        Ok([
            ("nested".to_string(), config::Value::new(None, config::ValueKind::String("{\"start\": 1, \"end\": 3}".to_string())),
            ("nested.end".to_string(), config::Value::new(None, config::ValueKind::String("5".to_string()))),
        ].into_iter().collect())
    }
}

This can result in deserializing 1..3 or 1..5 randomly.

Start by inspecting Source::collect and the config::Map type, then run the provided ExampleSource reproducer repeatedly to observe the overlapping overrides. Trace how collected values are processed and determine where ordering is lost. Done means overlapping values produce the same deserialized result consistently, with coverage for the 1..3 versus 1..5 case.