* Turned `core_file` into an implementation of `random_read_write`.
* Turned PNG errors into a standard error category.
* Added a helper for generating what look like derived classes on-the-fly.
Added more modern generic I/O interfaces with implementation backed by stdio, osd_file and core_file, replacing io_generic. Also replaced core_file's build-in zlib compression with a filter.
unzip.cpp, un7z.cpp: Added option to supply abstract I/O interface rather than filename.
Converted osd_file, core_file, archive_file, chd_file and device_image_interface to use std::error_condition rather than their own error enums.
Allow mounting TI-99 RPK from inside archives.
