Majority of warnings removed via RustFix
Two remain one being a long return type and another being a name
suggestion from Clippy `to_owned` for MaybeString
The `Spanned` struct was originally included in the docs as a re-export,
but rustdoc doesn't render re-exports if the module from which it's
exported isn't documented.
Since `Spanned` is the only public item inside the `spanned` module this
commit shows the struct as a normal item in the index, and moves the
example from the module docs to the struct docs.
