Support serde

[toyboot4e]

Implement serde for toecs::World and try hot reloading!

Use case

Run your toecs-based game from a host crate. Recompile the game and

1. Serialize the toecs-based game
2. Reload the toecs-based game library
3. Deserialize the last data

How to implement serde

ECS serde support may be one of the followings:

1. Directly serialize and deserialze the World
   This is not human-readable and not good for backward-compatibilities, but it would work out of the box.

2. Use intermediate data format such as DynamicScene in Bevy
   This is human-readable and better for backward-compatibilities, but there would be lots more things to consider.

This time I'll go with the 1: direct serde support.