Support geo types with georust/geo

[kestred]

This pull request implements part of #244. Supporting the default postgres geographic types with https://github.com/georust/rust-geo.

The types LINE and LSEG haven't been implemented yet because there is no obvious mapping in the current implementation of the geo crate. I hope to discuss this with the maintainer of that crate.

[kestred]

While implementing this, I had another idea; either an alternative or an additional implementation for geo types.

The geo types could be supported via tuples (eg. impl ToSql for (f64, f64)). This would allow queries to be written like:

let lat: f64 = 40.018782;
let lng: f64 = -105.278201;
db.execute("INSERT INTO places (loc) VALUES ($1)", &[(&lat, &lng)])

---

The type mappings using tuples COULD be:

Rust Type	Postgres Type
(f64, f64)	POINT
(f64, f64, f64)	LINE
((f64, f64), (f64, f64))	LSEG, BOX
[(f64, f64), ...] OR Vec<(f64, f64)>	PATH
[(f64, f64), ...] OR Vec<(f64, f64)>	POLYGON

[sfackler]

Thanks! Could you add some tests? See here for reference: https://github.com/sfackler/rust-postgres/tree/master/postgres/tests/types

[sfackler]

Is it going to be problematic that we're discarding this information?

[kestred]

This will not cause a database error to occur.

To recover this information, a user would need to SELECT the value of isopen(mypath).

The more interesting part of the issue is with ToSql where there are two different ways to create a path: an open path path '[(1,2),(3,4)]' and a closed path path '((1,2),(3,4))'. The PR handles this by always inserting an open path. To insert a closed path, you would need to write pclose($1) and let Postgres flip that bit during statement evaluation. This behavior is documented in the README.

I opted for that behavior because:

• The Polygon type is already very similar to a Closed path (see https://www.postgresql.org/docs/9.2/static/datatype-geometric.html#DATATYPE-GEO-TABLE)
• A LineString is treated like an Open path in most uses

[sfackler]

Thanks!