[Feature] Change execute signature

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**
What if we could express this
```rust
#[system]
pub mod game_loop {
pub fn execute(ctx: Context, delta_time: f32) -> Result {
Ok(ctx.accounts)
}

#[system_input]
pub struct Components {
pub game_state: GameState,
}
}
```
as
```rust
#[system]
impl System for GameLoop {
fn execute(&self, game_state: &mut GameState, delta_time: f32) -> Result<()> {
let in_case_we_need_to_access_context = self.context();
Ok(())
}
}
```

This would make the code more Rusty and we could also benefit from having the ability to define mutability.

Important note:

`Components` are always borrowed
System `Arguments` are always moved

Start by tracing the existing #[system] and #[system_input] handling described in the issue. Define how impl System with execute arguments should support borrowed Components, mutable references, moved arguments, and context access; the change is done when the proposed GameLoop form can express those semantics consistently.

Written by the indexing model from the issue text.