Merge nested `match`es

[mcarton]

We should have a lint that suggest to change

match foo {
    Ok(row) => match row {
        Some(pixels) => pixels,
        None => return Err(…),
    },
    Err(err) => return Err(…),
};

into

match foo {
    Ok(Some(pixels)) => pixels,
    Ok(None) => return Err(…),
    Err(...) => return Err(…),
}

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Copied from https://github.com/Manishearth/rust-clippy/issues/1245#issuecomment-250666810.

[oli-obk]

5 seconds!

[mcarton]

Héhé 😄