Interaction with Geo types

From @bluenote10's [comment](https://github.com/georust/geo/issues/380#issuecomment-573314264):

>I'm wondering if there is a good way to avoid the type conversion. Currently the robust crate interface relies on its own robust::Coord type. As far as I can see, computing orient2d on a geo::Coordinate requires to first duplicate the data into a robust::Coord, which seems unnecessary and a bit tedious. What is the best solution to that in Rust? Would it make sense to change the interface to orient2d(pa_x: f64, pa_y: f64, pb_x: f64, pb_y: f64, pc_x: f64, pc_y: f64)?

Several possibilities here:

- Switch to `geo_types::Coordinate` (I assume @frewsxcv didn't use it for a reason, but it may simply have been in order to get everything working)
- Write `From` impls for `Geo` types (and primitive types such as `(T, T)` and `[T; 2]`) to `robust::Coord`, and modify the functions to accept `Into` instead, with an explicit `into()` call inside the functions.

Either way, we'll probably want to land #3 first.

Review issue #3 and the current robust crate interface first. Compare the proposed geo_types::Coordinate and From/Into approaches, then establish which Geo types and conversions the API should support; done means an agreed approach for orient2d and related predicates.

Written by the indexing model from the issue text.