compiler: fix generation of default .str() methods in interpolation

vlib/compiler/tests/string_interpolation_struct_test.v

@@ -0,0 +1,21 @@
+// This file tests whether V can generate a convenience default .str() method
+// for a custom struct, when the developer has not defined one himself.
+// The .str() methods are used for string interpolation and for println() calls.
+
+struct Man {
+	name string
+	age int
+	interests []string
+}
+
+fn test_default_struct_string_interpolation(){
+	superman := Man{'Superman', 30, ['flying','fighting evil','being nice']}
+	s := '$superman'
+	assert s.contains('Man {')
+	assert s.contains('name: Superman')
+	assert s.contains('age: 30')
+	assert s.contains('interests: [')
+	assert s.contains('"being nice"')
+	assert s.contains('}')
+	//println(s)
+}