v2: more informative assert output; string interpolation formatting

vlib/compiler/tests/string_struct_interpolation_test.v

@@ -0,0 +1,25 @@
+
+struct Foo {
+	bar int
+mut:
+	str string
+}
+
+fn (f Foo) baz() string {
+	return 'baz'
+}
+
+fn test_string_method_interpolation() {
+	foo := Foo{}
+	s := 'baz=${foo.baz()}'
+	assert s == 'baz=baz'
+}
+
+fn test_adding_to_mutable_string_field() {
+	mut foo := Foo{10, 'hi'}
+	assert foo.bar == 10
+	assert foo.str == 'hi'
+	foo.str += '!'
+	eprintln( foo.str )
+	assert foo.str == 'hi!'
+}