Make polygon construction easier

[peterstace]

Make polygon construction easier

[peterstace]

Lot's of "helpers" could live in this new package. Some ideas are:

• Must variants that panic on error. The user doesn't need to handle errors for these if they're sure the inputs are valid (or are willing to risk a panic).
• Constructors for each geometry type directly from float64 coordinates.
• XYZ, XYM, and XYZM are out of scope. These have a pretty neiche usecases (and the regular []float64 -> Sequence -> Geoometry path is always available).
• Include Envelope constructors as well.
• The need to create regular polygons (approximating circles) tends to come up a lot. That may be useful to add too.