Add cached envelope benchmark

[urschrei]

• I agree to follow the project's code of conduct.
• I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

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[adamreichold]

Not really part of the change but since we are touching this file:

• circular_polygon is pub but I don't think it needs to be.
• create_random_polygons could return impl Iterator<Item=Polygon<f64>> to avoid unnecessary intermediate collections.

[urschrei]

@adamreichold like this?

[adamreichold]

@adamreichold like this?

Almost. The new bulk_load_complex_geom_cached benchmark still collects and then maps instead immediately doing something like

let cached: Vec<_> =
    create_random_polygons(DEFAULT_BENCHMARK_TREE_SIZE, 4096, SEED_1)
        .map(CachedEnvelope::new)
        .collect();