[geom] dropping support for interface types

[ear7h]

Support for things like Pointer, LineStringer, etc. Should be dropped from the general code base. They were added for interoperability, but support has been sparse and adds an extra cognitive load in switch x.(type) using code.

For interoperability, I propose a convert or interop package with contains the interfaces and and has a ToGeom function. A readme would also have an example on how to write a FromGeom function, and perhaps a FromGeomer interface which contains the necessary functions to convert from the geom types.