Don't access std::cout from Position::print()

Let the caller to decide where to redirect (cout or cerr) the
ASCII representation of the position. Rename the function to
reflect this.

Renamed also from_fen() and to_fen() to set() and fen() respectively.

No functional change.

src/benchmark.cpp

@@ -89,7 +89,7 @@ void benchmark(const Position& current, istream& is) {
       fens.assign(Defaults, Defaults + 16);
 
   else if (fenFile == "current")
-      fens.push_back(current.to_fen());
+      fens.push_back(current.fen());
 
   else
   {