SIMD instruction for node4

[rv64m]

Basic idea 💡

1. The order of node4 keys is not maintained.
2. For insert_child, append-only.
3. For remove_child, the child at the corresponding index is marked as none.
4. For get_child, use the SIMD.
5. For shrink_to_fit, use SIMD to find the only child that is not none.

Therefore, two basic operations need to be implemented using SIMD

1. Input key byte, return the index of the corresponding child
2. Find the index of the only child that is not empty