Remove duplicated Position object in UCIEngine

Also fixes searchmoves.

Drop the need of a Position object in uci.cpp.

A side note, it is still required for the static functions,
but these should be moved to a different namespace/class
later on, since sf kinda relies on them.

closes https://github.com/official-stockfish/Stockfish/pull/5169

No functional change

src/engine.cpp

@@ -24,6 +24,8 @@
 #include <string_view>
 #include <utility>
 #include <vector>
+#include <sstream>
+#include <iosfwd>
 
 #include "evaluate.h"
 #include "misc.h"
@@ -146,8 +148,6 @@ void Engine::save_network(const std::pair<std::optional<std::string>, std::strin
 
 // utility functions
 
-OptionsMap& Engine::get_options() { return options; }
-
 void Engine::trace_eval() const {
     StateListPtr trace_states(new std::deque<StateInfo>(1));
     Position     p;
@@ -158,4 +158,16 @@ void Engine::trace_eval() const {
     sync_cout << "\n" << Eval::trace(p, networks) << sync_endl;
 }
 
+OptionsMap& Engine::get_options() { return options; }
+
+std::string Engine::fen() const { return pos.fen(); }
+
+void Engine::flip() { pos.flip(); }
+
+std::string Engine::visualize() const {
+    std::stringstream ss;
+    ss << pos;
+    return ss.str();
+}
+
 }