Support UTF16 directly

This makes the API easier to use from javascript

src/runtime/utf16.c

@@ -0,0 +1,24 @@
+#include "runtime/utf16.h"
+
+int utf16_iterate(const uint8_t *string, size_t length, int32_t *code_point) {
+  uint16_t *units = (uint16_t *)string;
+  uint16_t unit = units[0];
+
+  if (unit < 0xd800 || unit >= 0xe000) {
+    *code_point = unit;
+    return 2;
+  }
+
+  if (unit < 0xdc00) {
+    if (length >= 4) {
+      uint16_t next_unit = units[1];
+      if (next_unit >= 0xdc00 && next_unit < 0xe000) {
+        *code_point = 0x10000 + ((unit - 0xd800) << 10) + (next_unit - 0xdc00);
+        return 4;
+      }
+    }
+  }
+
+  *code_point = -1;
+  return 2;
+}