Separate constructors for different dimensions of coordinates

[frewsxcv]

Have separate constructors if the caller wants to assume their data is a certain dimension. The hope is that the new constructors will be more performant since each coordinate doesn't require heap allocations. I'm not tied to these method names.

enum GeoJson<C> {
    Geometry(Geometry<C>),
    Feature(Feature<C>),
    FeatureCollection(FeatureCollection<C>),
}

impl<C> GeoJson<C> {
    /// Parse a string into `GeoJson`
    pub fn from_str(s: &str) -> Result<GeoJson<Vec<f32>>, Error> { ... }

    /// Parse a string into `GeoJson`. Coordinates are assumed to have two dimensions.
    /// If a non-two-dimensional coordinate is encountered, an error will be returned.
    pub fn from_str_2d(s: &str) -> Result<GeoJson<(f32, f32)>, Error> { ... }

    /// Parse a string into `GeoJson`. Coordinates are assumed to have three dimensions.
    /// If a non-three-dimensional coordinate is encountered, an error will be returned.
    pub fn from_str_3d(s: &str) -> Result<GeoJson<(f32, f32, f32)>, Error> { ... }
}

enum Value<C> {
    Point(C),
    MultiPoint(Vec<C>),
    ...
}