FEATURE: Support boolean and EMPTY as a format string variable.

Also add format string regression tests.

integration_tests/format_test.ucg

@@ -0,0 +1,4 @@
+assert |"hello @" % ("world") == "hello world"|;
+assert |"1 @ @" % (2, 3) == "1 2 3"|;
+assert |"@ or @" % (true, false) == "true or false"|;
+assert |"@" % (NULL) == "NULL"|;

src/build/compile_test.rs

@@ -73,3 +73,8 @@ fn test_concatenation() {
         "../../integration_tests/concatenation_test.ucg"
     ));
 }
+
+#[test]
+fn test_format() {
+    assert_build(include_str!("../../integration_tests/format_test.ucg"));
+}

src/build/mod.rs

@@ -276,6 +276,8 @@ impl From<Val> for String {
             Val::Int(ref i) => format!("{}", i),
             Val::Float(ref f) => format!("{}", f),
             Val::Str(ref s) => s.to_string(),
+            Val::Boolean(ref b) => format!("{}", b),
+            Val::Empty => "NULL".to_string(),
             val => format!("<{}>", val),
         }
     }