Verbatim-safe typography, ^-punctuation quoting, full-range ^UUUU^, polyglot html

Typographic transforms (---, quote pairs, ~) no longer touch verbatim
text: @c/@code/@source_listing content and ^'...'^ spans show exactly the
characters written. "^" before any punctuation character quotes it in
every target (the apostrophe excepted: ^' opens a literal span), with the
new :resolve option on @@@target declaring per-target renderings. The
^UUUU^ code-point form accepts 4-6 hex digits, the full Unicode range.
The html output and transform spellings are polyglot (XML-valid), in
preparation for an EPUB target. New suites: transform_test, character_test
(engine), typography_test (SKS).

(from dev 07ce5ea86a0a)

com/ktext.cpp

@@ -49,7 +49,7 @@ int main(int argc, char* argv[])
 
         if (input_text.empty() && input_filenames.empty()) {
             throw Argument_error(
-                "You must specify input filenames and/or text", Locator());
+                "You must specify input filenames and/or text", Locator::none());
         }
 
         auto [target, output_dir, output_basename, output_filename,