Write core image files.

[larsbrinkhoff]

Extend the file format code to make it possible to write out a core image. So far just the raw format.

I don't have a use case for this, but it could come in handy at some point.

Future directions: support output in more formats.

[larsbrinkhoff]

If PDUMP output is desirable, the core image needs additional information about page read/write status.

[larsbrinkhoff]

Implemented the basics. Exercises left to the pull request reader:

• Handle more meta data like assembly info (author, source, time).
• Write DMP in-core symbol table.