fix(core): fix an edge case for split_off operation for RrbVec

[arazabishov]

There is an edge case that brakes the push operation on RrbVec. Here are the steps to reproduce it:

• Create a new RrbVec instance exactly of the BRANCH_FACTOR * BRANCH_FACTOR size.
• Split it exactly at the following index: BRANCH_FACTOR * BRANCH_FACTOR - BRANCH_FACTOR.
• Try pushing a new item onto the new (right-hand side) vector.

A simple example for a vector that has a branching factor of 4:

• The in-memory representation of the following vector is going to be a complete and full tree: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15].
• Trying to split it at index 12 will produce two new vectors: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] and [12, 13, 14, 15].

The right hand size vector will have a full tail. The bug was that the tail was not pushed onto tree when full. When trying to push an ordinary item, 16 for example, it will overflow the vector and cause a panic.

[vercel]

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Name	Status	Preview	Updated (UTC)
pvec-rs	✅ Ready (Inspect)	Visit Preview	Oct 21, 2023 8:10pm