Allow parsing ItemMod in inner attribute

[nhynes]

Syn assumes that every ItemMod has an ident (i.e. pub mod $ident { ... }, but this is not the case when adding an inner attribute to a file mod:

#![feature(custom_inner_attributes, proc_macro_hygiene)]
#![MyCustomAttribute]

// ...

This PR remedies this by making ItemMod.ident an Option.

[dtolnay]

Thanks for the PR. Rustc does not support crate-level and module-level inner attributes, so I am not going to accept syntax tree changes based on what invalid tokens the current implementation happens to pass. The behavior of passing a mod with missing name should be considered an accident arising from the implementation of some other feature. Once there is an RFC or substantial discussion affirming some behavior as the intended behavior, then we can consider making syntax tree changes if necessary.