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 https://github.com/BurntSushi/rust-csv/pull/175