Reduce allocations in horizontal_water_movement system

[alice-i-cecile]

There are a large number of unnecessary allocations in the system, primarily in the addItion_map, removal_map and flow_map hashmaps and the hashmap returned by proposed lateral flow to neighbors.

Instead, we should add:

/// Cache of water to be added and removed from each tile during [`horizontal_water_movement`].
///
/// This is for performance reasons, as it allows us to reduce allocations by reusing the same space.
#[derive(Component, Default, Debug, PartialEq, Clone, Copy, Serialize, Deserialize)]
pub struct LateralWaterCache {
    /// The amount of water to be added to each tile.
    incoming: Volume,
    /// The amount of water to be added to each neighbor.
    outgoing: [Volume; 6],
}

to the WaterBundle.

Then, this should be mutated in place.

[alice-i-cecile]

This should probably wait on #905, as a very similar system is needed there for signal propagation.