Allow items to be anonymous

[catamorphism]

Right now, if we have an anonymous item, we use a dummy ident for its ident field in the AST. This should really be an option, since for example, anonymous foreign mods will soon be allowed.

[catamorphism]

Not critical for 0.6, de-milestoning

[catamorphism]

This seems to be obsolete, since we don't have anonymous foreign mods anymore.