Further API cleanup:

 * Remove SightlyRuntime interface
 * Rename SightlyRuntimeImpl to SightlyRuntime
 * Drop getSightlyRuntime() from RenderContext interface
   (but keep in RenderContextImpl and use RenderContextImpl in BaseRenderUnit.render)
 * Move SightlyRenderException to implementation
 * Move SightlyParsingException to implementation
 * Move RenderUnit to implementation