`join_k_path_into` leaves two values on one key when shortened keys collide

[adamv-symbolica]

LineListNode::drop_head_dyn (line_list_node.rs ~2610): when byte_cnt is shorter than both keys, the node is
reused with both keys shortened in place — with no check that the shortened keys are now equal. Two value
slots end up on the same key: a path with two values.

map = {aaaa, acaa}
map.write_zipper().join_k_path_into(3, false);
map.iter()      -> ["a"]      // iteration visits the path once
map.val_count() -> 2          // both slots are counted

Downstream, PR #31's recursive cata hits its Case 7 debug_assert!(key1.len() > 1) on such nodes (that is how
the edit program found it); validate_node does not reject two value slots with identical keys, only two
children. The child/child and value/child collisions are handled correctly.

Fix: when the shortened keys coincide and both slots are the same kind, merge them — pjoin the two values
(or the two children) into a single slot; a value and a child on one key is the legal representation and is left
alone. Consider also teaching validate_node to reject two value slots on one key.