Implement Area based on traits

[kylebarron]

• [x] I agree to follow the project's code of conduct.

• [ ] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

An exploration of https://github.com/georust/geo/issues/1113, based on top of #1114. Having a bunch of traits is verbose 😕 . Is there any way to group lots of traits as one? Wishing for something like

pub mod AreaTraits {
    pub use super::AreaPoint;
    pub use super::AreaLineString;
}

where importing AreaTraits would add all of the internal traits to scope. Guessing that doesn't exist, and probably for a good reason.

[frewsxcv]

I hope it's okay I converted this to a draft PR, since it's not really ready for a review until we figure out a plan for geo-traits