Initial ES modules support

lib/stdlib/string.js

@@ -1,5 +1,13 @@
+import {
+    __rt_number_to_string,
+    __rt_string_char_code_at,
+    __rt_string_concat,
+    __rt_string_from_char_code,
+    __rt_string_slice,
+} from "jsrt:native";
+
 // Converts any value to a string, like the spec's ToString
-function String(x) {
+export function String(x) {
     switch (typeof x) {
         case "string": return x;
         case "number": return __rt_number_to_string(x);
@@ -109,7 +117,7 @@ function clampIndex(i, len) {
 }
 
 // An index where a negative value counts from the end, clamped to [0, len]
-function relativeIndex(i, len) {
+export function relativeIndex(i, len) {
     return clampIndex(i < 0 ? i + len : i, len);
 }