[DDMD] Add ddmd manually ported source and build system support

@WalterBright @andralex @AndrejMitrovic @MartinNowak @9rnsr

Finally, the time has come to start moving forward with this.

This pull adds a new make target 'ddmd' which converts the source over and builds it with the just-build C++ dmd.

The most important file is `settings.json` which contains the type lists and mappings from C++ source to D source.  (type list generation is  now mostly automated)

To build, just run `make ddmd`.

Some of the source remains stubbed out (response, libmach, etc) and some files are missing headers and other cleanup, but this is a purely additive change and will hopefully encourage some more devs experiment with ddmd and test their pull requests against it.
Will do.