bevy_asset, bevy_state, and bevy_log feature gates

[Totobird-Creations]

Title pretty much explains it.
This adds the bevy_asset, bevy_state, and bevy_log crate features which gate the bevy features with the same names.

[mintlu8]

That's too many feature flags than what I would like. Is there any reason bevy_defer would be used with such a barebones version of bevy?

I can understand removing bevy_asset, since its a somewhat large dependency.

[Totobird-Creations]

bevy_asset is definitely the main one. Like you said, large dependency, and some headless applications don't care to have it.

bevy_state, I don't use in a lot of my Bevy projects. One I'm working on is a non-game, long-running application that has no reason to keep track of a "global state". bevy_defer is still being used in this project because there are several async tasks which need access to the world.

Gating bevy_log is debatable. I decided to include it for completeness.

The point of this PR is to allow people to disable anything they don't need, that isn't essential. The bevy library does this for the bevy_* crates.
You're probably correct in saying that most people will keep these flags enabled, but having the option is the point.

[mintlu8]

The point of this PR is to allow people to disable anything they don't need, that isn't essential. The bevy library does this for the bevy_* crates. You're probably correct in saying that most people will keep these flags enabled, but having the option is the point.

The problem is more feature flags makes things harder to test since you can't really spam all the feature flag combinations in ci.