http::write_some_file_body, http::async_write_some_file_body

We need dedicated interface like `async_write_some_file` which calls the natively-enhanced algorithms (such as using `::TransmitFile` on Windows, or `::sendfile` on POSIX). This will eliminate users' uncertainty of whether or not the enhanced algorithm will be used. These functions guarantee the algorithms will be used, unless the native enhancement is not available on the target platform.