define SmallHashMap in spacetimedb_data_structures (#4136)

# Description of Changes

Defines `SmallHashMap` in `spacetimedb_data_structures`.
The data structure is a hybrid map that is backed by a vector and linear
scan for the first `M` elements.
Then it falls back into a normal hash map.
For the first `N` elements, where `N < M`, the vector is stored inline.
That is, the vector is a `SmallVec<[(K, V); N>`.
The structure is optimized for access patterns where the keys and values
are accessed together,
therefore, keys and values are not stored separately.

# API and ABI breaking changes

None, just additive.

# Expected complexity level and risk

1

# Testing

A comprehensive proptest suite is added for the datastructure.

Author: Centril

Cargo.lock

@@ -14,11 +14,15 @@ serde = ["dep:serde", "hashbrown/serde"]
 spacetimedb-memory-usage = { workspace = true, optional = true, default-features = false }
 ahash.workspace = true
 crossbeam-queue.workspace = true
+either.workspace = true
 hashbrown.workspace = true
 nohash-hasher.workspace = true
 serde = { workspace = true, optional = true }
 smallvec.workspace = true
 thiserror.workspace = true
 
+[dev-dependencies]
+proptest.workspace = true
+
 [lints]
 workspace = true

crates/data-structures/Cargo.toml

@@ -6,3 +6,4 @@ pub mod map;
 pub mod nstr;
 pub mod object_pool;
 pub mod slim_slice;
+pub mod small_map;