Add optional serde feature (enabled by default)

[LucaCappelletti94]

Makes the serde dependency optional (enabled by default) by gating all serde-related functionality behind a serde feature flag.
This PR will make it possible to use csv in other crates which offer the serde feature flag, and makes the groundwork for adding an optional std feature flag which will be necessary to use csv with other crates which work with solely alloc.

Fixes #412, and replaces the old closed PR #175

[BurntSushi]

Thanks! It might be some time before I have the bandwidth to review and get this merged.

[LucaCappelletti94]

Thanks! It might be some time before I have the bandwidth to review and get this merged.

I am fixing the last failing CI, do let me know if I can facilitate in any way the PR.

[LucaCappelletti94]

While I am at it, I will also do a round of clippy code-smell fixes.

Note that the changes in the code are strictly limited to adding serde feature gates, nothing fancier than that. I have added also a run in the CI for the no-default-feature case, so everything is looking a-okay right now.

[LucaCappelletti94]

Assuming that the current PR should be more or less merged as-is (given the triviality of it) I will get started with the no-std one, which I need for the aforementioned apache/datafusion-sqlparser-rs#2077