Implement save APIs and provide an mdmerge tool that composes an image from deltas and writes it to disk instead of dumping to console.

[jkoritzinsky]

With these APIs, we can now implement writing an image out to disk or into memory.

The mdmerge tool produces a raw metadata file, not a PE-enveloped file. As a result, mddump (in this repo) is one of the few decent tools that can read it.

The work to fully construct a composite delta image in a PE envelope that can be run requires additional work. For example, including valid RVAs is difficult and requires a more comprehensive implementation and CLI interface, so I deferred on this work as it would likely be as complex if not moreso than the actual metadata saving logic.

Most of the code in mdmerge is a direct copy of mddump.