add functions to redirect input/output

those two multiply the output into a file:

void redirect_output(const char *filename);
void redirect_output(std::string const &filename)

those two read everything from a file:

redirect_input(const char *filename);
redirect_input(std::string const& filename);

both can be resetted using:

void reset_input();
reset_output();

example.cpp

@@ -27,6 +27,9 @@ signed main() {
   a.foo();
   a.bar(3,4,"hi");
 
+  print("this should not be in the output file");
+  redirect_output("example.out");
+
   auto [i, j] = read<int, int>();
   print(i, j);
 
@@ -68,4 +71,12 @@ signed main() {
   print(map<int, string>{{3,"three"},{1,"one"}});
   print(tuple{unordered_map<int, string>{{3,"three"},{1,"one"}}});
   print(unordered_set<int>{3,1,4});
+
+  redirect_input("example.in");
+  print("reading from example.in:", read_string());
+  redirect_input("example.in");
+  print("reading from example.in:", read_string());
+  print("this is the last line on example.out");
+  reset_output();
+  print("this is stdout only");
 }