[phobos] ModuleInfo - class to struct?

[Walter Bright]

I'd like to change the ModuleInfo from a class to a struct. This will 
eliminate the reference in each module to the ModuleInfo vtbl[], which 
winds up pulling in everything. I don't think there is any use for 
ModuleInfo for polymorphic behavior.

This should only impact druntime's source code (and will require 
everything to be recompiled). Have I missed anything?