File reads aren't thread safe

[Tim-Evans-Seequent]

The OMF code is incorrectly assuming that std::fs::File::try_clone returns an entirely separate file object. It does not: the returned object uses the same file handle and any seek affects all clones. This breaks OMF file reading in multi-threaded environments.

There are a couple of options for fixing this:

• Implement a file deep copy. This would duplicate the underlying Unix file descriptor or Windows file handle using OS-specific code. I've written this elsewhere and it works.

• Create a new implementation of std::fs::Read, wrapping Arc<File> and implementing clone that keeps the positions separate. This would again require OS-specific code, as Unix and Windows have slightly different implementations of the required calls: read_at vs. seek_read.

Either of these options could be added to the existing omf::file::sub_file::SubFile type. The second might make more sense there as it's already tracking the file position to implement the sub-file.