Support for serde

[TehPers]

While trying to make a keybinding config, I came across the issue that bevy_input::KeyCode doesn't implement Serialize or Deserialize and there is no feature to do so. I think it would be nice to have support for serde (behind a feature) for types which users of bevy may want to serialize, such as KeyCode and MouseButton.

[cart]

Yeah this makes sense to me!

[erlend-sh]

Keep in mind there’s also https://github.com/not-fl3/nanoserde and https://github.com/dtolnay/miniserde.

[cart]

I think its worth giving the "minimal serde" implementations a fair shake, given how much they improved macroquad compiles. But I dont think we'll see similar clean compile time improvements without fully removing syn + quote dependencies from the tree. They run pretty deep in many rust crates, so this will likely be very hard. In general, while serde does have a compile time cost, it is the ecosystem standard and it feels "worth it" to be aligned and have the additional flexibility it gives. We also heavily rely on erased_serde for bevy_property. But if they can yield significant compile time wins, then I'm definitely open to using them.

Short term, I don't see much harm in adding additional serde impls, given how prevalent it is in our codebase (and our dependencies' code bases).

[zicklag]

For compile times you may want to investigate cargo watt as a way to pre-compile the proc macros to WASM, preventing the need to compile them when building bevy.