Add cross-platform GitHub Actions CI test workflows

.github/workflows/fmt.yml

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+on: [push, pull_request]
+
+name: Rustfmt check
+
+jobs:
+  fmt:
+    name: rustfmt
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout source
+        uses: actions/checkout@v2
+      - name: Run rustfmt check
+        run: |
+          rustup component add rustfmt
+          cargo fmt --version
+          cargo fmt -- --check || (echo "Please reformat your code with 'cargo fmt' (version $(cargo fmt --version))"; false)

.github/workflows/linuxci.yml

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+on: [push, pull_request]
+
+name: GNU/Linux CI
+
+jobs:
+  check:
+    name: GNU/Linux CI
+    runs-on: ubuntu-latest
+    steps:
+      - name: Cache cargo registry
+        uses: actions/cache@v1
+        with:
+          path: ~/.cargo/registry
+          key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+      - name: Cache cargo index
+        uses: actions/cache@v1
+        with:
+          path: ~/.cargo/git
+          key: ${{ runner.os }}-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+      - name: Cache cargo build
+        uses: actions/cache@v1
+        with:
+          path: target
+          key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+
+      - uses: actions/checkout@v2
+      - name: Install latest stable toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+
+      - name: Run cargo test
+        uses: actions-rs/cargo@v1
+        with:
+          command: test

.github/workflows/macosci.yml

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+on: [push, pull_request]
+
+name: macOS CI
+
+jobs:
+  check:
+    name: macOS CI
+    runs-on: macos-latest
+    steps:
+      - name: Cache cargo registry
+        uses: actions/cache@v1
+        with:
+          path: ~/.cargo/registry
+          key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+      - name: Cache cargo index
+        uses: actions/cache@v1
+        with:
+          path: ~/.cargo/git
+          key: ${{ runner.os }}-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+      - name: Cache cargo build
+        uses: actions/cache@v1
+        with:
+          path: target
+          key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+
+      - uses: actions/checkout@v2
+      - name: Install latest stable toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+
+      - name: Run cargo test
+        uses: actions-rs/cargo@v1
+        with:
+          command: test

.github/workflows/windowsci.yml

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+on: [push, pull_request]
+
+name: Windows CI
+
+jobs:
+  check:
+    name: Windows CI
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install latest stable toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+
+      - name: Run cargo test
+        uses: actions-rs/cargo@v1
+        with:
+          command: test