Write files on CTFE

[andre2007]

I might have a solution for the missing write files on CTFE issue. Or more precise, with next version of Dub it will be possible.

• Add a new function to the shared object library which returns the metadata of the modules to be wrapped. (Either as JSON or better using structs). e.g. here https://github.com/symmetryinvestments/autowrap/blob/master/csharp/source/autowrap/csharp/boilerplate.d#L290
• Add to dub.sdl of autowrap a post generate command: "postGenerateCommands": ["$DUB postgen.d $ROOT_PACKAGE_DIR"],
• Create in autowrap folder a new file postgen.d. Every dub package which has a dependency to autowrap will automatically call the postgen.d application while executing dub build.

The app postgen.d gets the folder path to the dub package, which has a dependency to autowrap, as console argument. In addition it reads the 3 new environment variables DUB_ROOT_PACKAGE_TARGET_TYPE, DUB_ROOT_PACKAGE_TARGET_PATH and DUB_ROOT_PACKAGE_TARGET_NAME. Therefore it knows the shared object file which was built and can call the metadata function. The metadata can now be used to create the `cs´ files.

The benefit is, you do not longer need 2 different configurations (wrap and emit). Just executing dub build will create the SO and also the cs files are automatically created.

The drawback is, you need a very recent dub version (The 3 new environment variable were just added to master).