[Feature] Change execute signature

[notdanilo]

Is your feature request related to a problem? Please describe. What if we could express this

#[system]
pub mod game_loop {
    pub fn execute(ctx: Context<Components>, delta_time: f32) -> Result<Components> {
        Ok(ctx.accounts)
    }

    #[system_input]
    pub struct Components {
        pub game_state: GameState,
    }
}

as

#[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