Clean up marker generics for systems (#7789)

# Objective

While we use `#[doc(hidden)]` to try and hide marker generics from the user, these types reveal themselves in compiler errors, adding visual noise and confusion.

## Solution

Replace the `AlreadyWasSystem` marker generic with `()`, to reduce visual noise in error messages. This also makes it possible to return `impl Condition<()>` from combinators.

For function systems, use their function signature as the marker type. This should drastically improve the legibility of some error messages.  
The `InputMarker` type has been removed, since it is unnecessary.

crates/bevy_ecs/src/schedule/condition.rs

@@ -137,7 +137,7 @@ pub mod common_conditions {
  event::{Event, EventReader},
  prelude::{Component, Query, With},
  schedule::{State, States},
- system::{In, IntoPipeSystem, ReadOnlySystem, Res, Resource},
+ system::{In, IntoPipeSystem, Res, Resource},
  };
 
  /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
@@ -373,9 +373,7 @@ pub mod common_conditions {
  /// #
  /// # fn my_system() { unreachable!() }
  /// ```
- pub fn not<Marker>(
- condition: impl Condition<Marker>,
- ) -> impl ReadOnlySystem<In = (), Out = bool> {
+ pub fn not<Marker>(condition: impl Condition<Marker>) -> impl Condition<()> {
  condition.pipe(|In(val): In<bool>| !val)
  }
 }

crates/bevy_ecs/src/system/exclusive_function_system.rs

@@ -4,8 +4,8 @@ use crate::{
  component::ComponentId,
  query::Access,
  system::{
- check_system_change_tick, ExclusiveSystemParam, ExclusiveSystemParamItem, In, InputMarker,
- IntoSystem, System, SystemMeta,
+ check_system_change_tick, ExclusiveSystemParam, ExclusiveSystemParamItem, In, IntoSystem,
+ System, SystemMeta,
  },
  world::{World, WorldId},
 };
@@ -181,7 +181,7 @@ pub trait ExclusiveSystemParamFunction<Marker>: Send + Sync + 'static {
 macro_rules! impl_exclusive_system_function {
  ($($param: ident),*) => {
  #[allow(non_snake_case)]
- impl<Out, Func: Send + Sync + 'static, $($param: ExclusiveSystemParam),*> ExclusiveSystemParamFunction<((), Out, $($param,)*)> for Func
+ impl<Out, Func: Send + Sync + 'static, $($param: ExclusiveSystemParam),*> ExclusiveSystemParamFunction<fn($($param,)*) -> Out> for Func
  where
  for <'a> &'a mut Func:
  FnMut(&mut World, $($param),*) -> Out +
@@ -209,7 +209,7 @@ macro_rules! impl_exclusive_system_function {
  }
  }
  #[allow(non_snake_case)]
- impl<Input, Out, Func: Send + Sync + 'static, $($param: ExclusiveSystemParam),*> ExclusiveSystemParamFunction<(Input, Out, $($param,)* InputMarker)> for Func
+ impl<Input, Out, Func: Send + Sync + 'static, $($param: ExclusiveSystemParam),*> ExclusiveSystemParamFunction<fn(In<Input>, $($param,)*) -> Out> for Func
  where
  for <'a> &'a mut Func:
  FnMut(In<Input>, &mut World, $($param),*) -> Out +

crates/bevy_ecs/src/system/function_system.rs

@@ -325,10 +325,8 @@ pub trait IntoSystem<In, Out, Marker>: Sized {
  fn into_system(this: Self) -> Self::System;
 }
 
-pub struct AlreadyWasSystem;
-
 // Systems implicitly implement IntoSystem
-impl<In, Out, Sys: System<In = In, Out = Out>> IntoSystem<In, Out, AlreadyWasSystem> for Sys {
+impl<In, Out, Sys: System<In = In, Out = Out>> IntoSystem<In, Out, ()> for Sys {
  type System = Sys;
  fn into_system(this: Self) -> Sys {
  this
@@ -362,8 +360,6 @@ impl<In, Out, Sys: System<In = In, Out = Out>> IntoSystem<In, Out, AlreadyWasSys
 /// }
 /// ```
 pub struct In<In>(pub In);
-#[doc(hidden)]
-pub struct InputMarker;
 
 /// The [`System`] counter part of an ordinary function.
 ///
@@ -611,7 +607,7 @@ pub trait SystemParamFunction<Marker>: Send + Sync + 'static {
 macro_rules! impl_system_function {
  ($($param: ident),*) => {
  #[allow(non_snake_case)]
- impl<Out, Func: Send + Sync + 'static, $($param: SystemParam),*> SystemParamFunction<((), Out, $($param,)*)> for Func
+ impl<Out, Func: Send + Sync + 'static, $($param: SystemParam),*> SystemParamFunction<fn($($param,)*) -> Out> for Func
  where
  for <'a> &'a mut Func:
  FnMut($($param),*) -> Out +
@@ -638,7 +634,7 @@ macro_rules! impl_system_function {
  }
 
  #[allow(non_snake_case)]
- impl<Input, Out, Func: Send + Sync + 'static, $($param: SystemParam),*> SystemParamFunction<(Input, Out, $($param,)* InputMarker)> for Func
+ impl<Input, Out, Func: Send + Sync + 'static, $($param: SystemParam),*> SystemParamFunction<fn(In<Input>, $($param,)*) -> Out> for Func
  where
  for <'a> &'a mut Func:
  FnMut(In<Input>, $($param),*) -> Out +