Minor docs

[ci skip]
WKT use geomFromPtr ctor
Make some coord seq names unexported
Change API for points and linestrings

This changeset introduces a Coord type and a more Go-like way of creating new
points and linestrings from slices of coordinates. It effectively hides the
CoordSeq (and underlying C API) from users of the package.
Change polygon constructor to use coord slices

The main polygon constructor now takes a slice of Coord for its shell and a
variable number of Coord slices for its holes.
gofmt
Change to polygon API and coord slice

* Add Coords() to return a slice of Coord, to replace CoordSeq()
* Rename ExteriorRing to Shell()
* Replace NInteriorRing() and InteriorRing(int) with Holes(), returning a slice
  of *Geometry
Change test case to use new Coords() method
Merge branch 'api'