impl ToWkt for inner geo_type variants, allow non-geotypes to impl ToWkt

[michaelkirk]

• [x] I agree to follow the project's code of conduct.
• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

• [x] I ran cargo fmt

Draft because it's based on #89, so please review that first. update: merged!

The user benefit here is the implementation of ToWkt directly on the inner geometry enums so...

instead of:

let point: geo_types::point!(x: 1.0, y: 2.0);
let geo_geometry = geo_types::Geometry::from(point);
let string = geo_geometry.wkt_string();
assert_eq!("POINT(1 2)", string);

you can do:

let point: geo_types::point!(x: 1.0, y: 2.0);
let string = point.wkt_string();
assert_eq!("POINT(1 2)", string);

I also wanted to detangle ToWkt from geo-types, so that (hypothetical) non-geo-types users could implement it.

The majority of the lines changed are in ToWkt for non-geotypes, but it's a misleading large diff (sorry). Largely it's a cut and paste job: I cut the geo-types specific code from towkt.rs and pasted it into the new geo_types_to_wkt.rs.

For symmetry, and to better reflect it's functionality, I renamed the conversions module to geo_types_from_wkt.

[michaelkirk]

Ok, this is ready for review!

[michaelkirk]

Anyone available to review — maybe @categulario or @urschrei ?

[michaelkirk]

bors r=urschrei

[bors[bot]]

Build succeeded:

• wkt