Re: NamespaceDecl declarations/namespaces

this patch allows you to loop over gdb.NamespaceDecl's from the global_namespace, and get at declarations, and some various things needed along the way.

I wasn't quite sure on where to put the functions i added to gcc-python-tree.c or if they are named inappropriately.

one strange thing i suppose is the alias_of/unalias duality, in gccint it says: Do not attempt to use `cp_namespace_decls' for a namespace which is an alias. Instead, follow `DECL_NAMESPACE_ALIAS' links until you reach an ordinary, non-alias, namespace, and call `cp_namespace_decls' there.

alias_of calls DECL_NAMESPACE_ALIAS once, and returns None if not an alias while unalias does the looping, and returns self if not an alias. turns out DECL_NAMESPACE_ALIAS seems to always return the same alias as looping over it. rather than the actual alias (in alias of alias cases), thus the looping doesn't seem to be doing much, but I kept both around for the None vs Self behaviour. if this behaves differently on various gcc's i'll have to do something about the test. though, now that I think about it maybe I should get rid of alias_of, and turn it into is_alias returning a BOOL, let me know what you prefer. couldn't think of a better name than unalias, but didn't particularly like it.