depend on serde_derive directly

[jakobhellermann]

If no crate enables the derive feature of serde, then serde and serde_derive can compile in parallel, and anything depending only on serde can compile while serde_derive is not yet done.
This can save up to 5 seconds of compilation, depending on whether this helps the critical chain and if no other crate enables the feature.

This cuts the critical chain of bevy compilation from serde_derive -> serde -> hashbrown -> indexmap -> petgraph -> bevy_utils -> bevy_reflect to serde_derive -> glam -> bevy_math -> bevy_reflect which is faster.

[bitshifter]

Is glam the only thing in bevy that depends on serde_derive?

[jakobhellermann]

The only one that depends on serde { features = ["derive"] } was ron (ron-rs/ron#441) and bevy itself.

[bitshifter]

Don't need this one now.