The copy_file operation implementation has been inlined into the
detail::copy_file function. The part that copies the file body has been
extracted to a separate function, so that addition of specialized copy
implementations later is possible.
Added copy_options enum, which reflects the enum from C++20. Currently,
only overwrite_existing option is supported. Other options will be added
later.
The old enum copy_option is deprecated in favor of copy_options.
Updated docs to reflect recent changes to copy_file behavior.
