Implement missing APIs

[PsiACE]

The following API should be implemented:

Current expectations

• Entries API, used by ckb-util
• rayon support, just like indexmap or griddle
• compare with linked_hash_set

Need to discuss

• Some API consistent with the behavior of or_insert and or_insert_with in linked-hash-map

Anyone can implement these by hand, and add relevant discussions below.