utf8: add utf8_to_utf16() helper

src/basic/utf8.h

@@ -25,7 +25,12 @@ char *utf8_escape_invalid(const char *s);
 char *utf8_escape_non_printable(const char *str);
 
 size_t utf8_encode_unichar(char *out_utf8, char32_t g);
+size_t utf16_encode_unichar(char16_t *out, char32_t c);
+
 char *utf16_to_utf8(const char16_t *s, size_t length /* bytes! */);
+char16_t *utf8_to_utf16(const char *s, size_t length);
+
+size_t char16_strlen(const char16_t *s);
 
 int utf8_encoded_valid_unichar(const char *str);
 int utf8_encoded_to_unichar(const char *str, char32_t *ret_unichar);