writef: add %c format specifier support

src/hulk/writef.d

@@ -26,7 +26,15 @@ size_t writef(string s, va_list args, ch_out_fn ch_out)
     {
         if (escape)
         {
-            if (c == 'x')
+            if (c == 'c')
+            {
+                ulong ch;
+                va_arg(args, ch);
+                ch_out(cast(ubyte)ch);
+                length_written++;
+                escape = false;
+            }
+            else if (c == 'x')
             {
                 ulong v;
                 va_arg(args, v);