Provide central way to ignore all derive(Serialize, Deserialize) attributes

[dtolnay]

See the discussion at #288 (comment).

Instead of requiring #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))] on every type, there should be a central way to disable all of the derive(Serialize, Deserialize) attributes in a crate and eliminate the slow-to-compile dependency. This could take the form of a noop feature for serde_macros and/or serde_codegen, or a separate crate serde_noop.

[dtolnay]

Compile time of serde_derive is less of an issue now and cfg_attr is an adequate solution when that is really what you want.