lang: Add mutex around Expr String/Value/SetValue calls

The golang race detector complains about some unimportant races, and as
a result, this patch adds some mutexes to prevent these test failures.
We actually lock more than necessary, because a more accurate version
would be more time consuming to implement. Secondarily, it's likely that
in the future we replace this function graph algorithm with something
that is guaranteed to be glitch-free and supports back pressure.

lang/lang_test.go

@@ -93,6 +93,7 @@ func runInterpret(code string) (*pgraph.Graph, error) {
 		return errwrap.Wrapf(lang.Close(), "close failed")
 	}
 
+	// we only wait for the first event, instead of the continuous stream
 	select {
 	case err, ok := <-lang.Stream():
 		if !ok {
@@ -103,7 +104,7 @@ func runInterpret(code string) (*pgraph.Graph, error) {
 		}
 	}
 
-	// run artificially without the entire engine
+	// run artificially without the entire GAPI loop
 	graph, err := lang.Interpret()
 	if err != nil {
 		err := errwrap.Wrapf(err, "interpret failed")