lang: Pass through the Fs and the FsURI

This should give us options as to how a function should interact with an
FS. I feel like it's cleaner to go through the World API, and passing in
the FsURI lets us do that, but I passed in the Fs at the same time in
case it's useful for some reason. I think using it is a boundary
violation, but it's just a hunch. Does anything break when we move from
one deploy to the next?

lang/interfaces/ast.go

@@ -142,6 +142,10 @@ type Data struct {
 	// system to manage file resources or other aspects.
 	Fs engine.Fs
 
+	// FsURI is the fs URI of the active filesystem. This is useful to pass
+	// to the engine.World API for further consumption.
+	FsURI string
+
 	// Base directory (absolute path) that the running code is in. If an
 	// import is found, that's a recursive addition, and naturally for that
 	// run, this value would be different in the recursion.

lang/interfaces/func.go

@@ -115,6 +115,16 @@ type NamedArgsFunc interface {
 // TODO: Consider combining this with the existing Data struct or more of it...
 // TODO: Do we want to add line/col/file values here, and generalize this?
 type FuncData struct {
+	// Fs represents a handle to the filesystem that we're running on. This
+	// is necessary for opening files if needed by import statements. The
+	// file() paths used to get templates or other files from our deploys
+	// come from here, this is *not* used to interact with the host file
+	// system to manage file resources or other aspects.
+	Fs engine.Fs
+
+	// FsURI is the fs URI of the active filesystem. This is useful to pass
+	// to the engine.World API for further consumption.
+	FsURI string
 
 	// Base directory (absolute path) that the running code is in. This is a
 	// copy of the value from the Expr and Stmt Data struct for Init.