Introduce wcs2string_callback

This is like wcs2string, but instead of returning a std::string, it invokes
a user-supplied function with each converted character.

The idea is to allow interleaved conversion and output.

src/wcstringutil.cpp

@@ -8,6 +8,7 @@
 #include <locale>
 
 #include "common.h"
+#include "flog.h"
 
 wcstring_range wcstring_tok(wcstring &str, const wcstring &needle, wcstring_range last) {
     using size_type = wcstring::size_type;
@@ -196,3 +197,7 @@ wcstring join_strings(const wcstring_list_t &vals, wchar_t sep) {
     }
     return result;
 }
+
+void wcs2string_bad_char(wchar_t wc) {
+    FLOGF(char_encoding, L"Wide character U+%4X has no narrow representation", wc);
+}