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/unification_test.go

@@ -836,6 +836,7 @@ func TestUnification1(t *testing.T) {
 			t.Logf("\n\ntest #%d: AST (before): %+v\n", index, ast)
 
 			data := &interfaces.Data{
+				// TODO: add missing fields here if/when needed
 				Debug: testing.Verbose(), // set via the -test.v flag to `go test`
 				Logf: func(format string, v ...interface{}) {
 					t.Logf(fmt.Sprintf("test #%d", index)+": ast: "+format, v...)