std::shared_ptr isn't strictly necessary here and is only ever used in
contexts where the object doesn't depend on being shared. This also
makes the interface more flexible, as it's possible to create a
std::shared_ptr from a std::unique_ptr (std::shared_ptr has a
constructor that accepts a std::unique_ptr), but not the other way
around.
Eliminates the need to rebuild some source files if the file_util header
ever changes. This also uncovered some indirect inclusions, which have
also been fixed.
