`hash2curve`: Bringing breaking changes to ease maintainability & usability.

There are a few changes I would like to bring to hash2curve, mainly to reduce the amount of code the crate has and make it independent of the elliptic-curve crate.

Motivations

1. Reducing the amount of code is always a plus IMHO - makes the crate more maintainable & approachable.
2. elliptic-curve is almost unused as a dependency, but brings a lot of transitive dependencies (see below for comparison).
3. Some functions use out parameters and I would like to change that where possible (Expander and hash_to_field).

Changes

• Change GroupDigest to something like a Suite trait.

/// A hash to curve suite.
///
/// <https://www.rfc-editor.org/rfc/rfc9380.html#name-suites-for-hashing>
pub trait Suite {
    const ID: &'static str;

    type Point: MapToCurve;
    type SecurityLevel: Unsigned;
    type ExpandMsg: ExpandMsg<Self::SecurityLevel>;

    fn hash_from_bytes(msg: &[&[u8]], dst: &[&[u8]]) -> Option<Self::Point> {
        // ...
    }

    fn encode_from_bytes(msg: &[&[u8]], dst: &[&[u8]]) -> Option<Self::Point> {
        // ...
    }
}

• MapToCurve will be bound by Group instead of CurveArithmetic.
• All places that return elliptic_curve::Result<_> will return Option instead (elliptic_curve::Error was already as ZST).
• Make hash_to_field output Array<F, C> for F: FromOkm and C: ArraySize instead of taking &mut [F] as out parameter.
• Make ExpandMsg return Iterator<Item = u8> instead of an Expander (This may cause a slight performance regression, will be benchmarked before it is added).

Dependency tree with & without elliptic-curve

With elliptic-curve

hash2curve v0.14.0-rc.0
├── digest v0.11.0-rc.0
│   ├── block-buffer v0.11.0-rc.4
│   │   └── hybrid-array v0.3.1
│   │       ├── typenum v1.18.0
│   │       └── zeroize v1.8.1
│   └── crypto-common v0.2.0-rc.3
│       └── hybrid-array v0.3.1 (*)
├── elliptic-curve v0.14.0-rc.10
│   ├── base16ct v0.2.0
│   ├── crypto-bigint v0.7.0-pre.6
│   │   ├── hybrid-array v0.3.1 (*)
│   │   ├── num-traits v0.2.19
│   │   │   [build-dependencies]
│   │   │   └── autocfg v1.5.0
│   │   ├── rand_core v0.9.3
│   │   ├── subtle v2.6.1
│   │   └── zeroize v1.8.1
│   ├── ff v0.14.0-pre.0
│   │   ├── rand_core v0.9.3
│   │   └── subtle v2.6.1
│   ├── group v0.14.0-pre.0
│   │   ├── ff v0.14.0-pre.0 (*)
│   │   ├── rand_core v0.9.3
│   │   └── subtle v2.6.1
│   ├── hybrid-array v0.3.1 (*)
│   ├── rand_core v0.9.3
│   ├── subtle v2.6.1
│   └── zeroize v1.8.1
├── ff v0.14.0-pre.0 (*)
└── subtle v2.6.1

Without:

hash2curve v0.14.0-rc.0
├── digest v0.11.0-rc.0
│   ├── block-buffer v0.11.0-rc.4
│   │   └── hybrid-array v0.3.1
│   │       └── typenum v1.18.0
│   └── crypto-common v0.2.0-rc.3
│       └── hybrid-array v0.3.1 (*)
├── ff v0.14.0-pre.0
│   ├── rand_core v0.9.3
│   └── subtle v2.6.1
├── group v0.14.0-pre.0
│   ├── ff v0.14.0-pre.0 (*)
│   ├── rand_core v0.9.3
│   └── subtle v2.6.1
└── subtle v2.6.1

Start by reviewing the hash2curve crate's existing GroupDigest, MapToCurve, hash_to_field, Expander, and ExpandMsg APIs, along with its elliptic-curve dependency. Done means implementing the proposed breaking API changes, replacing Result with Option where specified, removing the dependency, and benchmarking the iterator-based ExpandMsg approach; no specific files or tests are named.