Switch to foldhash-portable and add generic BuildHasher support

- Replace xxhash-rust with foldhash-portable (portable feature)
- Add S: BuildHasher generic parameter to Filter and Builder
- Add _with_hasher constructors and with_hasher() for swapping hashers
- Document T: Hash stability caveats and custom hasher usage
- Update tests for new hash algorithm

Author: arthurprs

Cargo.toml

@@ -19,7 +19,7 @@ jsonschema = ["schemars"]
 serde = ["dep:serde", "dep:serde_bytes"]
 
 [dependencies]
-xxhash-rust = { version = "0.8.12", features = ["xxh3"] }
+foldhash-portable = { version = "0.2.0", features = ["portable"] }
 serde = { optional = true, version = "1", features = ["derive"] }
 serde_bytes = { optional = true, version = "0.11" }
 schemars = { optional = true, version = "1" }

README.md

@@ -42,7 +42,9 @@ for i in 0..1000 {
 
 ### Hasher
 
-The hashing algorithm used is [xxhash3](https://crates.io/crates/xxhash-rust) which offers both high performance and stability across platforms.
+Methods accepting `T: Hash` are provided for convenience using [foldhash-portable](https://crates.io/crates/foldhash-portable), which offers high performance and stability across platforms. Note that a fixed seed is used (no DoS resistance) and `#[derive(Hash)]` output is [not guaranteed stable](https://github.com/hoxxep/portable-hash#whats-wrong-with-the-stdhash-traits) across Rust compiler versions.
+
+`Filter` supports a custom `BuildHasher` via its `S` type parameter (similar to `HashMap`). Use `Filter::new_with_hasher()` and related constructors. The hasher is not serialized (reconstructed via `S: Default`), and filters being merged must use the same hasher.
 
 ### Filter size