[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.
I'm not so sure it should.  It makes perfect sense when building with the just-built dmd, but not so much when using the system dmd.  In the version I'm using on the autotester I've removed all dependencies on `$(DC)`.