Description of Changes

Migrating clockworklabs/com.clockworklabs.spacetimedbsdk#318 since we are merging that repo into this one.

Annotate rows with their hashes. This is halfway to the old solution of annotating code with their serialized forms; it speeds up insertion into BTreeIndexes while saving a byte array allocation per-row.
(Wait, why do we hash when inserting into BTreeIndexes, you ask? It's because we use a HashSet to store the multiple rows corresponding to a non-unique key.)

This saves work on the main thread repeatedly hashing values. Once we parallelize message processing, this work will be spread out over multiple threads too.

Also adds a data type SmallHashSet<T> for use in BTreeIndexes. This is a struct that can store at most one element without allocating. This gives dramatic allocation reduction on initial connection -- it seems that most rows in BTreeIndexes don't have any other rows with the same key.
Then I manually specialized this, since it's only used on one type -- PreHashedRow. using SmallHashSetOfPreHashedRow instead of SmallHashSet<PreHashedRow> allows us to inline some methods and pass around fewer pointers while hashing things, which is probably good.

API and ABI breaking changes

None

Expected complexity level and risk

Testing

I will test:

• blackholio
• bitcraft