[wip] Initial pass at LineString constructor

[urschrei]

Ref #56
This checks whether the vector being used has one element, and fails if it does.
The error is currently just a String. Is this OK?

[urschrei]

Trying out from() and into() for constructing LineStrings using common source data structures;

• Vec<[T; 2]>>
• <Vec<(T, T)>>
  where T is Float

This works well imo, but where ::new() returns a Result(), and can catch the major stumbling block (1-element sequence), from() and into() can't, and will panic if you do something like
let ls: LineString = vec![[1.0, 2.0]].into()
I'm not sure to what extent this is in conflict with the guidance that the From() and Into() traits must not fail (Is an unrecoverable panic (apart from catch_unwind) a failure?) – TryInto will be stable soon-ish (I think), so we could wait for that.

[frewsxcv]

I like where this is going. I'm wondering, from an API perspective, it makes sense to make the internal field on the LineString no longer pub. In concrete terms, this would be that construction would be done with LineString::new (which could also be called LineString::from_points) and if they want to access the underlying points, we could provide methods like:

• as_slice(&self) -> &[Point]
• into_vec(self) -> Vec<Point>

Though this might be a tangential conversation.

[urschrei]

Oh, I like that idea. Will see how it works out…

[frewsxcv]

Opened a similar PR: #168