Support use as inner attributes

[Anders429]

Each attribute could potentially be used as an inner attribute.

#![doc_item::since("1.28.0")]
#![doc_item::docbox(content = "...")]

The above code block could be used at a crate-level to define crate-level since and item-info values.

As far as I can tell, the main challenge will be identifying whether the attribute is in an inner or outer position. The way to tell seems to be to check for pub mod { ... }. As I understand, pub mod {} is not valid in any other context, and is only wrapped around the full crate's code. As far as being used as an inner-attribute in other contexts, that may require more investigation surrounding what is passed in as the item TokenStream.

Note that this usage would require nightly, with custom_inner_attributes enabled.