refactor!: Remove feature mock

The so-called “MockMMR” is a historic artifact that is unused in direct
dependencies. Turn it into a type that is accessible only from unit
tests and remove the feature `mock` to slim down the public API.

Replace `hashbrown::HashSet` with `std::collections::HashSet`, which
amounts to the same thing but allows removing the direct dependency on
`hashbrown`.

Other changes, only relevant for developers of twenty-first:
- rename the “MockMMR” to the more canonical “ArchivalMMR”.
- reflect the internals more accurately by renaming the collection of
  all nodes from “leafs” to “nodes”
- remove helper struct `MyVec<_>`
- replace some helper functions with sane initialize functions
- reduce visibility of some internal helper functions
- rework documentation in some places

Author: jan-ferdinand

twenty-first/Cargo.toml

@@ -15,10 +15,6 @@ readme.workspace = true
 keywords = ["polynomial", "merkle-tree", "post-quantum", "algebra", "tip5"]
 categories = ["cryptography", "mathematics"]
 
-[features]
-# include mock module
-mock = []
-
 [dev-dependencies]
 blake3 = "1.5.5"
 bincode = "1.3.3"
@@ -38,7 +34,6 @@ features = ["user-hooks"]
 arbitrary = { version = "1", features = ["derive"] }
 bfieldcodec_derive = "0.7"
 get-size2 = { version = "0.3", features = ["derive"] }
-hashbrown = "0.15"
 hex = "0.4.3"
 itertools = "0.14"
 num-bigint = { version = "0.4", features = ["serde"] }

twenty-first/src/lib.rs

@@ -6,9 +6,6 @@ pub mod math;
 pub mod prelude;
 pub mod util_types;
 
-#[cfg(any(test, feature = "mock"))]
-pub mod mock;
-
 // This is needed for `#[derive(BFieldCodec)]` macro to work consistently across crates.
 // Specifically:
 // From inside the `twenty-first` crate, we need to refer to `twenty-first` by `crate`.
@@ -64,7 +61,6 @@ pub(crate) mod tests {
         implements_usual_auto_traits::<math::lattice::kem::SecretKey>();
         implements_usual_auto_traits::<math::lattice::kem::PublicKey>();
         implements_usual_auto_traits::<math::lattice::kem::Ciphertext>();
-        implements_usual_auto_traits::<mock::mmr::MockMmr>();
         implements_usual_auto_traits::<util_types::sponge::Domain>();
         implements_usual_auto_traits::<util_types::mmr::mmr_accumulator::MmrAccumulator>();
         implements_usual_auto_traits::<math::zerofier_tree::Branch<BFieldElement>>();

twenty-first/src/mock/mmr/mod.rs

@@ -5,5 +5,8 @@ pub mod mmr_trait;
 pub mod shared_advanced;
 pub mod shared_basic;
 
+#[cfg(test)]
+pub(crate) mod archival_mmr;
+
 const TOO_MANY_LEAFS_ERR: &str =
     "internal error: Merkle Mountain Ranges should have at most 2^63 leafs";