Proj Rework: Clean up Core Gem

[keithdoggett]

As part of a proj and coord_sys refactoring project:

We need to remove references to proj4 in the core gem so that they can be properly decoupled.

• SimpleMercatorProjector: Subclass CoordinateTransform and implement those methods.
• Factory instantiation: Rely on the srid keyword to define the crs, or a new coord_sys parameter that accepts a CoordSys object. We could have a combination where if SRID is given, but not coord_sys, we try to make a coord_sys with an available adapter.
• Proj4Projector: Likely remove, but we could create a new Projector class that subclasses CT, although it probably makes more sense for the projector param on geographic factories to just take a CT and ensure that the source_cs is 4326.
• SRSDatabase Module: Remove.
• CoordSys#supported? & #check!: Remove.
• Feature#cast: Use the CoordSys information from each factory to create a CT that will transform the geometry.