Revise signature::SnapshotEntries iterator

[mbrt]

Instead of implementing Iterator for SnapshotEntries, it's better to implement IntoIterator for SnapshotEntries, &SnapshotEntries, &mut SnapshotEntries. This allows to add to SnapshotEntries these methods independently:

• entry(&self, id: EntryId) -> Entry;
• into_display(self) -> SnapshotEntriesDisplay.