Proximity Map implementation with support for incremental edits.

[nicktobey]

Based on #8408, now with additional functionality for incremental changes to indexes.

This is a large-scale PR merging several features into main, all designed for supporting vector indexes.

Vector Index Nodes

1defec9 adds a new message/node type: the vector index node. This message stores a node in a Merkle tree index whose structure is based on some distance measure in a multi-dimensional space: at each level, keys are arranged such that a key is closer to its parent key than any other key in the parent node.

One consequence of this design is that it's not possible to put a hard limit on the number of keys contained in each node. We can control the mean node size, but there's always a non-zero chance that a node will be large enough to break our usual encoding scheme (which uses 16-bit ints to store message offsets). To address this, the vector index node uses 32-bit ints to store message offsets instead of the 16 bits used by other node types.

Proximity Map

A ProximityMap is a new implementation of Dolt's Map, a data structure built on Merkle trees that maps key bytestrings to value bytestrings. The ProximityMap is backed by a tree of vector index nodes, allowing it to perform an approximate nearest neighbor search.

Proximity Maps resemble other Prolly Maps, but have the following invariants:

• Each key must be convertible to a vector. Typically, the key is a val.Tuple, and the vector is the first value in that tuple.
• The keys are arranged in the tree such that, for each of a key's parent keys (the keys that appear on the path from the root to the key), the key is closer to that parent key than any of the parent key's siblings.
• The keys in a node are sorted lexographically (note that this is not necessarily the same ordering as the tuple that the key represents), except for the first key which matches its direct parent.

Notably, while the keys of an individual node are sorted, walking all of a vector indexes keys in standard iteration order will not be sorted.

28b7065 and 6b91635 contain the bulk of the ProximityMap implementation.

The bulk of the changes are in these three commits. Each of the other commits is a smaller self-contained change necessary to support vector indexes.

[coffeegoddd]

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