Mirage 3 and package split
- Update to Mirage 3
- Port to jbuilder
- Split the original OPAM package into two: vhd-format for the core library and vhd-format-lwt for the Lwt-specific parts, with corresponding ocamlfind libraries vhd-format and vhd-format-lwt, and top-level modules Vhd_format and Vhd_format_lwt, respectively.