Implement postgis::Point for geo_types::Point (and other traits) in the geo-postgis crate

[frewsxcv]

Instead of providing conversion, we should also implement the traits for our types

[michaelkirk]

This would have to go in the geo-types crate right?

Alternatively, could we eliminate this crate if postgis got a geo-types feature?

Relatedly, I've been musing on if it's a better strategy to prefer putting impl Foo for geo_types:F in geo-types (behind --features=foo) vs trying to upstream the impl Foo for geo_types::F into the third party Foo crate?

When we bump the geo-types version, it seems like it would be easier if we could also update all the impls in the same repo/branch rather than having to chase down a bunch of disparate PR's in third party repositories later. This would also give us better and easier insight into the ramifications of potential API changes by forcing the changer to reconcile the integrations.

We'd also be potentially taking on some upgrade churn to keep third party deps up to date in geo-types, so it'd probably be best to be somewhat defensive about which integrations are "officially supported".

I know this is somewhat of a tangent to the original issue, but it seems relevant to the outcome of resolving this issue.