Revise using string interpolation.

tests/call_fn.rs

@@ -81,7 +81,7 @@ fn test_call_fn_args() -> Result<(), Box<EvalAltResult>> {
     let ast = engine.compile(
         r#"
             fn hello(x, y, z) {
-                if x { "hello " + y } else { y + z }
+                if x { `hello ${y}` } else { y + z }
             }
         "#,
     )?;

tests/modules.rs

@@ -315,7 +315,7 @@ fn test_module_from_ast() -> Result<(), Box<EvalAltResult>> {
         
             // Final variable values become constant module variable values
             foo = calc(foo);
-            hello = "hello, " + foo + " worlds!";
+            hello = `hello, ${foo} worlds!`;
 
             export
                 x as abc,