Store dimensionality in parsed WKT objects

[kylebarron]

WKT strings can store a dimensionality even while being EMPTY. E.g. these are valid:

POINT EMPTY
POINT Z EMPTY
POINT M EMPTY
POINT ZM EMPTY

Currently, the parsed representation does not store this. It only stores a coordinate or a sequence of coordinates, depending on the type:

wkt/src/types/linestring.rs

Line 23 in b2f7399

pub struct LineString<T: WktNum>(pub Vec<Coord<T>>);

I think it would be a worthwhile change to also store the dimensionality as part of the geometry. E.g.:

pub struct LineString<T: WktNum> {
    coords: Vec<Coord<T>>,
    dim: Dimension,
}

This would be nice for two reasons:

1. It would solve the empty geometry problem by storing the dimension even when there are no coordinates.
2. It would make it easier to implement the dim method for each geometry trait, because it wouldn't have to recurse into the first coordinate.

Unfortunately, this would be a breaking change because the .0 element of the geometry types is public.