Implement iteration over report attachments in mutable fashion

As requested by Micks_Ketches in the discord:

- A `ReportAttachmentsIterMut<'a>` iterator to mirror `ReportAttachmentsIter<'a>` that does the obvious thing
- A `ReportAttachmentMut<'a, A>` struct to mirror `ReportAttachmentRef<'a, A>` that does the obvious thing
- A `ReportAttachments::iter_mut(&mut self) -> ReportAttachmentsIterMut<'a>` function that does the obvious thing