sha1: implement collision detection and mitigation (#566)

This implements sha1 collision detection, including rehashing for mitigation.
As the code is 1-1 based on the version that git uses, the mitigation
hashes should match as well.

## Limitations

Can only be used with the pure software implementation, asfaiu. The
reason for this is, that the algorithm needs access to all intermediary
states, and so processing 4 rounds at once through the intrinsics will
screw things up.
For that reason I have made it it's own implementation, instead of
adapting the existing `compress` implementations.

It might be possible to add support for the "simpler" assembly
implementations that do round for round processing, but I think this
could be a follow up in the future, if this is too slow for these
platforms.


## Prior art 

- Paper: https://marc-stevens.nl/research/papers/C13-S.pdf
- C reference implementation: https://github.com/cr-marcstevens/sha1collisiondetection
- C2Rust translation: https://gitlab.com/sequoia-pgp/sha1collisiondetection/

.github/workflows/sha1-checked.yml

@@ -0,0 +1,155 @@
+name: sha1-checked
+
+on:
+  pull_request:
+    paths:
+      - ".github/workflows/sha1-checked.yml"
+      - "sha1-checked/**"
+      - "Cargo.*"
+  push:
+    branches: master
+
+defaults:
+  run:
+    working-directory: sha1-checked
+
+env:
+  RUSTFLAGS: "-Dwarnings"
+  CARGO_INCREMENTAL: 0
+
+jobs:
+  set-msrv:
+    uses: RustCrypto/actions/.github/workflows/set-msrv.yml@master
+    with:
+        msrv: 1.72.0
+
+  # Builds for no_std platforms
+  build:
+    needs: set-msrv
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+          - ${{needs.set-msrv.outputs.msrv}}
+          - stable
+        target:
+          - thumbv7em-none-eabi
+          - wasm32-unknown-unknown
+          - loongarch64-unknown-linux-gnu
+    steps:
+      - uses: actions/checkout@v4
+      - uses: RustCrypto/actions/cargo-cache@master
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.rust }}
+          targets: ${{ matrix.target }}
+      - uses: RustCrypto/actions/cargo-hack-install@master
+      - run: cargo hack build --target ${{ matrix.target }} --each-feature --exclude-features default,std
+
+  minimal-versions:
+    uses: RustCrypto/actions/.github/workflows/minimal-versions.yml@master
+    with:
+        working-directory: ${{ github.workflow }}
+
+  # Linux tests
+  linux:
+    needs: set-msrv
+    strategy:
+      matrix:
+        include:
+          # 32-bit Linux/x86
+          #- target: i686-unknown-linux-gnu
+          #  rust: ${{needs.set-msrv.outputs.msrv}}
+          #  deps: sudo apt update && sudo apt install gcc-multilib
+          #- target: i686-unknown-linux-gnu
+          #  rust: stable
+          #  deps: sudo apt update && sudo apt install gcc-multilib
+
+          # 64-bit Linux/x86_64
+          - target: x86_64-unknown-linux-gnu
+            rust: ${{needs.set-msrv.outputs.msrv}}
+          - target: x86_64-unknown-linux-gnu
+            rust: stable
+
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: RustCrypto/actions/cargo-cache@master
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.rust }}
+          targets: ${{ matrix.target }}
+      - uses: RustCrypto/actions/cargo-hack-install@master
+      - run: ${{ matrix.deps }}
+      - run: cargo hack test --feature-powerset
+
+  # macOS tests
+  macos:
+    needs: set-msrv
+    strategy:
+      matrix:
+        rust:
+          - ${{needs.set-msrv.outputs.msrv}}
+          - stable
+
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: RustCrypto/actions/cargo-cache@master
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.rust }}
+          targets: x86_64-apple-darwin
+      - run: cargo test --no-default-features
+      - run: cargo test
+      - run: cargo test --all-features
+
+  # Windows tests
+  windows:
+    needs: set-msrv
+    strategy:
+      matrix:
+        include:
+          # 64-bit Windows (GNU)
+          - target: x86_64-pc-windows-gnu
+            toolchain: ${{needs.set-msrv.outputs.msrv}}
+          - target: x86_64-pc-windows-gnu
+            toolchain: stable
+
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: RustCrypto/actions/cargo-cache@master
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.toolchain }}
+          targets: ${{ matrix.target }}
+      - uses: msys2/setup-msys2@v2
+      - run: cargo test --target ${{ matrix.target }}
+
+  # Cross-compiled tests
+  cross:
+    strategy:
+      matrix:
+        rust:
+          - 1.72.0
+          - stable
+        target:
+          - aarch64-unknown-linux-gnu
+          - powerpc-unknown-linux-gnu
+        features:
+          - default
+
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+      # Cross mounts only current package, i.e. by default it ignores workspace's Cargo.toml
+        working-directory: .
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ./.github/actions/cross-tests
+        with:
+          rust: ${{ matrix.rust }}
+          package: ${{ github.workflow }}
+          target: ${{ matrix.target }}
+          features: ${{ matrix.features }}