Make serde an optional dependency

From the example I have the impression that serde is not actually needed in order to use this crate. If I am correct, then it would be better to specify serde as an optional dependency and compile it only if the user wants it. It would be like this:

`Cargo.toml`
```toml
[dependencies]
serde = {version = "1.0.55", optional = true}

[features]
default = ["serde"]
```

This way, if the user does not want serde, all what it has to do is to put this in his `Cargo.toml`

```toml
[dependencies.csv]
version = "1"
default-features = false
```

Start by reviewing Cargo.toml and the crate's example to confirm whether serde is required for the example or the library. Configure serde as an optional dependency with the proposed default feature behavior, then verify that the crate still builds with default features and with default-features = false.

Written by the indexing model from the issue text.