cgen: support .str() for maps with non string keys (#7806)

vlib/builtin/map_test.v

@@ -517,6 +517,8 @@ fn test_rune_keys() {
 	assert m[`!`] == 2
 	m[`@`] = 7
 	assert m.len == 3
+	println(m)
+	assert '$m' == '{`!`: 2, `%`: 3, `@`: 7}'
 	
 	mut a := []rune{}
 	for k, v in m {
@@ -588,3 +590,9 @@ fn test_eq() {
 		}
 	}
 }
+
+fn test_non_string_key_map_str() {
+	assert {23: 4}.str() == '{23: 4}'
+	assert {`a`: 12, `b`: 13}.str() == '{`a`: 12, `b`: 13}'
+	assert {23: 'foo', 25: 'bar'}.str() == "{23: 'foo', 25: 'bar'}"
+}