Move to_char() and to_string() to notation

Where they better belong.

Also, this removes '#include <string>' from types.h, which reduces the amount of code to compile (every
translation unit includes types.h).

No functional change.

src/position.cpp

@@ -30,6 +30,7 @@
 #include "rkiss.h"
 #include "thread.h"
 #include "tt.h"
+#include "notation.h"
 
 using std::string;