Better with attribute

[fredszaq]

Requires #535, (I will mark this PR as non-draft once #535 is merged; for now, you will see changes from both PRs in the diff). The actual changes are in this commit d24745a.

Synopsis

Allow with atribute to contain syn::ExprCall and syn::ExprClosure in addition to syn::Path

Solution

This uses the same approach as what was done for the #[from_str(error(...))]. I extracted the type used to parse the content of the attribute to one named Callable.

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)