Rework tree insert to remove unneeded parent update

[declanvk]

Current in the insert_unchecked function we will unconditionally update the parent node if it was found during search: https://github.com/declanvk/blart/blob/main/src/nodes/operations/insert.rs#L372-L373

Based on experience writing the delete_unchecked function, we can choose to conditionally write to update the parent node only when the insert function creates a new inner node.