Support UTF16 directly

This makes the API easier to use from javascript

src/runtime/utf16.h

@@ -0,0 +1,20 @@
+#ifndef RUNTIME_UTF16_H_
+#define RUNTIME_UTF16_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <stdint.h>
+#include <stdlib.h>
+
+// Analogous to utf8proc's utf8proc_iterate function. Reads one code point from
+// the given string and stores it in the location pointed to by `code_point`.
+// Returns the number of bytes in `string` that were read.
+int utf16_iterate(const uint8_t *string, size_t length, int32_t *code_point);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // RUNTIME_UTF16_H_