Group improvements (#28)

* Implement `ConstantTimeEq` for `Group` and `Group::Scalar`

* Changed `expand_message_xmd` `dst` parameter to `GenericArray<u8, D>`

* Remove allocation for `dst_prime` in `expand_message_xmd`

* Remove allocation for `msg_prime` and `b` in `expand_message_xmd`

* Remove unnecessary references in `expand_message_xmd`

* Return `GenericArray` from `expand_message_xmd`

* Oxidize loop in `expand_message_xmd`

* Remove allocation for `b` in `expand_message_xmd`

* Remove allocation for `xor`

* De-duplicate code in `expand_message_xmd`

* Remove unnecessary features

* Make ristretto optional and export all backends

* Make `rand` optional

* Fix wrong documentation

* Document `rand` feature

* Remove empty line

* Fix rustfmt

* Fix testing with other backends

* Iterate by value in `expand_message_xmd`

* Fix typo

* Rename `ristretto_x` features to `ristretto255_x`

* Remove `rand` re-export

* Improve `expand_message_xmd` loop

* Remove unnecessary `#[doc(cfg())]`

* Remove unnecessary `pub(crate)` for `mod p256`

* Fix `p256` only build

* Rename `serialize` feature to `serde`

* Fix missing `Cargo.toml` update

.github/workflows/main.yml

@@ -13,11 +13,11 @@ jobs:
       fail-fast: false
       matrix:
         backend_feature:
-          - u64_backend
-          - u32_backend
-          - p256,u64_backend
+          - ristretto255_u64
+          - ristretto255_u32
+          - p256,ristretto255_u64
         frontend_feature:
-          - serialize
+          - serde
         toolchain:
           - stable
           - 1.51.0
@@ -57,16 +57,18 @@ jobs:
           # for any no_std target
           - thumbv6m-none-eabi
         backend_feature:
-          - u64_backend
-          - u32_backend
-          - p256,u64_backend
+          -
+          - --features ristretto255_u64
+          - --features ristretto255_u32
+          - --features p256
         frontend_feature:
-          - serialize
+          -
+          - --features serde
     steps:
       - uses: actions/checkout@v2
       - uses: hecrj/setup-rust-action@v1
       - run: rustup target add ${{ matrix.target }}
-      - run: cargo build --verbose --target=${{ matrix.target }} --no-default-features --features ${{ matrix.frontend_feature }} --features ${{ matrix.backend_feature }}
+      - run: cargo build --verbose --target=${{ matrix.target }} --no-default-features ${{ matrix.frontend_feature }} ${{ matrix.backend_feature }}
 
 
   clippy: