Batch map generation example

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Adds an example in the world crate to mass-generate maps (with multiple threads!).

Usage

cargo run -F cli --example batch_generate batch --threads 8 batch_config.ron

Example config:

(
    scale: ( start: 2.0, end: 2.5 ),
    size: ( 10, 10 ),
    kind: Circle,
    erosion_quality: ( start: 1.0, end: 1.0 ),
)

This creates a maps/ folder with the following structure:

maps/
 {seed}.png
 {seed}.ron
 {seen}.bin # If --save-bin was passed to the generate command
 ...

To regenerate a map from a {seed}.ron file run: cargo run -F cli --example [--save-bin] regenerate [--erosion-quality 1.0] {seed}.ron