Add `num_rings` or `rings_count` to `Polygon`

[frewsxcv]

[edpft]

Is this just 1 + length of interiors vector or am I missing something?

[callpraths]

Thanks for adding starter bugs for the likes of me :) Is there interest in adding this?

A few clarifications would be useful:

1. Naming: I'd vote for rings_count to go with existing coords_count (PostGIS uses ST_NRings, geos crate has get_num_interior_rings and nothing that includes the exterior ring).
2. Should this be added directly to struct impl(s)?
3. It should probably be added to: Polygon, MultiPolygon, Rect (always 1) and Triangle (always 1).

[callpraths]

I want to answer 2 with: Add geo::algorithm::RingsCount trait, even though it will only be implemented for a few of the core types. There are plenty of examples where algorithms only apply to subset of types (e.g. FrechetDistance only applies to LineString).

[frewsxcv]

Resolved in https://github.com/georust/geo/pull/1145