Move operations of an `ExecutableDocument` into a new struct

[SimonSapin]

• doc.anonymous_operation → doc.operations.anonymous
• doc.named_operations → doc.operations.named
• doc.get_operation() → doc.operations.get()
• doc.get_operation_mut() → doc.operations.get_mut()
• doc.insert_operation() → doc.operations.insert()
• doc.all_operations() → doc.operations.iter()

This change makes get_mut() borrow only doc.operations instead of the entire document, making it possible to also mutate doc.fragments during that mutable borrow.