Fix stack overflow when converting GeometryCollection scalar to geo

[kylebarron]

As described in a contained comment:

We can't implement both

impl From<GeometryCollection<'_>> for geo::GeometryCollection

and

impl From<GeometryCollection<'_>> for geo::Geometry

because of this problematic blanket impl
(https://github.com/georust/geo/blob/ef55eabe9029b27f753d4c40db9f656e3670202e/geo-types/src/geometry/geometry_collection.rs#L113-L120).

Thus we need to choose either one or the other to implement.

If we implemented only for geo::Geometry, then the default blanket impl for
geo::GeometryCollection would be wrong because it would doubly-nest the
GeometryCollection:

GeometryCollection(
    [
        GeometryCollection(
            GeometryCollection(
                [
                    Point(
                        Point(
                            Coord {
                                x: 0.0,
                                y: 0.0,
                            },
                        ),
                    ),
                ],
            ),
        ),
    ],
)

Therefore we must implement only for geo::GeometryCollection

Closes #979

[kylebarron]

It looks like a new version of clippy was released. I'll put up a PR to fix those new lints once I'm off this plane or tomorrow morning.

[BlakeOrth]

lgtm

Thanks for the quick turn around on the bug report!