Add :xml subPackage for dlangui to use

[rikkimax]

Grim isn't too happy that undead isn't building with GDC, this extracts it so it can be used by dlangui until a better solution is found.

[rikkimax]

I should mention that I didn't test this... but ugh it should be ok

[anon17]

AIU undead.xml compiles with gdc just fine (it depends only on phobos), it's undead.stream that doesn't compile, and dub compiles all files in a package. The package won't compile until all files in the package compile.

[ibuclaw]

AIU undead.xml compiles with gdc just fine (it depends only on phobos), it's undead.stream that doesn't compile,

Correct, as that old module relies on the discouraged use of RTTI based varargs.

[schveiguy]

Once my PR is merged, this should be closed. We could probably close it now.

[schveiguy]

I think this can be closed now.