stats: preparatory change of the implementation of %c

This now holds the scc_info while processing the %c sequence, so that
using options we will soon be able to specify which SCC to count.

* spot/twaalgos/stats.hh, spot/twaalgos/stats.cc (printable_scc_info):
New class.
(state_printer): Use it for %c.
* spot/misc/formater.hh: Add move assignment.
* bin/common_aoutput.hh, bin/common_aoutput.cc: Use printable_scc_info
for %C.
* tests/core/format.test: Add a quick test case to make sure nothing
changed.

spot/misc/formater.hh

@@ -71,6 +71,13 @@ namespace spot
       return *this;
     }
 
+    printable_value&
+    operator=(T&& new_val)
+    {
+      val_ = std::move(new_val);
+      return *this;
+    }
+
     virtual void
     print(std::ostream& os, const char*) const override
     {