Update map_iteratort to use a shared_ptr for current

This allows mapping into move only types. This was not previously
possible, because the copy constructor of `map_iteratort` copied the
value pointed to by its unique pointer in its `current` field. This can
also be considered to be more correct, because copying an iterator
should be expected to yield two copies, which point to the same value,
rather than two copies, which point to two separate values.

Author: thomasspriggs

src/util/range.h

@@ -48,7 +48,7 @@ class map_iteratort
     PRECONDITION(underlying != underlying_end);
     ++underlying;
     if(underlying != underlying_end)
-      current = util_make_unique<outputt>((*f)(*underlying));
+      current = std::make_shared<outputt>((*f)(*underlying));
     return *this;
   }
 
@@ -80,16 +80,7 @@ class map_iteratort
       f(std::move(f))
   {
     if(this->underlying != this->underlying_end)
-      current = util_make_unique<outputt>((*this->f)(*this->underlying));
-  }
-
-  map_iteratort(const map_iteratort &other)
-    : underlying(other.underlying),
-      underlying_end(other.underlying_end),
-      f(other.f)
-  {
-    if(other.current != nullptr)
-      current = util_make_unique<outputt>(*other.current.get());
+      current = std::make_shared<outputt>((*this->f)(*this->underlying));
   }
 
 private:
@@ -101,7 +92,7 @@ class map_iteratort
 
   /// Stores the result of \c f at the current position of the iterator.
   /// Equals nullptr if the iterator reached \c underlying_end.
-  std::unique_ptr<outputt> current = nullptr;
+  std::shared_ptr<outputt> current = nullptr;
 };
 
 /// Iterator which only stops at elements for which some given function \c f is