gapi: Plumb through a URI mechanism

This is at least a stop-gap until we redo the whole filesystem API mess.
I think golang is partly to blame because they don't have proper API's
merged yet.

gapi/gapi.go

@@ -120,6 +120,13 @@ type Next struct {
 	Err  error // if something goes wrong (use with or without exit!)
 }
 
+// InfoResult is some data that a GAPI can return on request.
+type InfoResult struct {
+	// URI is the FS URI that we pass around everywhere.
+	// TODO: can this be deprecated?
+	URI string
+}
+
 // GAPI is a Graph API that represents incoming graphs and change streams. It is
 // the frontend interface that needs to be implemented to use the engine.
 type GAPI interface {
@@ -132,6 +139,9 @@ type GAPI interface {
 	// Init initializes the GAPI and passes in some useful data.
 	Init(*Data) error
 
+	// Info returns some data about the GAPI implementation.
+	Info() *InfoResult
+
 	// Graph returns the most recent pgraph. This is called by the engine on
 	// every event from Next().
 	Graph() (*pgraph.Graph, error)