Save hierarchy to file

[gmlarumbe]

It should be possible to save the extracted hierarchy buffer to a file for future navigation. This feature could be useful for some purposes.

One simple way would be declaring verilog-ext-hierarchy-twidget-nav-mode/verilog-ext-hierarchy-outshine-nav-mode as major modes instead of minor modes, with an associated file extension (e.g. .hier) for auto-mode-alist.