Simplify system piping and make it more flexible

[JoJoJet]

Objective

• Currently, it is not possible to call .pipe on a system that takes any input other than ().
• The IntoPipeSystem trait is currently very difficult to parse due to its use of generics.

Solution

Remove the IntoPipeSystem trait, and move the pipe method to IntoSystem.

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Changelog

• System piping has been made more flexible: it is now possible to call .pipe on a system that takes an input.

Migration Guide

The IntoPipeSystem trait has been removed, and the pipe method has been moved to the IntoSystem trait.

// Before:
use bevy_ecs::system::IntoPipeSystem;
schedule.add_systems(first.pipe(second));

// After:
use bevy_ecs::system::IntoSystem;
schedule.add_systems(first.pipe(second));

[JoJoJet]

Not quite sure what to put for the migration guide. While this is technically an API breakage, I can't image a case in which anyone would actually have their code broken to use as an example.

[JoJoJet]

I took @james7132's suggestion of merging IntoPipeSystem with IntoSystem. Note that the module hierarchy no longer makes much sense, so we should reorganize it. In particular, IntoSystem shouldn't be in function_system.rs anymore, and system_piping.rs probably doesn't need to exist.

I'm saving these changes for a follow-up so the diffs won't be horrible, as they tend to be when moving files around.

[alice-i-cecile]

I took @james7132's suggestion of merging IntoPipeSystem with IntoSystem. Note that the module hierarchy no longer makes much sense, so we should reorganize it. In particular, IntoSystem shouldn't be in function_system.rs anymore, and system_piping.rs probably doesn't need to exist.

I'm saving these changes for a follow-up so the diffs won't be horrible, as they tend to be when moving files around.

Totally agree on both the need for reorganization and to do those in separate PRs that we merge ASAP.

[JoJoJet]

Totally agree on both the need for reorganization and to do those in separate PRs that we merge ASAP.

Sounds like a plan :)