Change the `copy_to_file` method

[Kerollmops]

Currently, the Env::copy_to_file doesn't take a file as an argument but a path, and the Env::copy_to_fd is too raw. Therefore, it is not easy to use the Env::copy_to_file along with the tempfile crate.

I want to propose a new API for the Env::copy_to_file method that takes a &File as argument:

pub fn copy_to_file(&self, file: &mut File, option: CompactionOption) -> Result<()>;

However, I noticed and forgot that the cursor position of the file descriptor is not resetting correctly. It seems that the file must be closed and reopened, which is an issue. The proposed API could not be released if this issue is still present. I need to investigate.