CI: Add initial github actions

Copy the workflow from `rust-bitcoin` and strip it down.

.github/workflows/rust.yml

@@ -0,0 +1,76 @@
+on:
+  push:
+    branches:
+      - master
+      - 'test-ci/**'
+  pull_request:
+
+name: Continuous integration
+
+jobs:
+  Stable:
+    name: Test - stable toolchain
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v4
+      - name: Checkout Toolchain
+        # https://github.com/dtolnay/rust-toolchain
+        uses: dtolnay/rust-toolchain@stable
+      - name: Running test script
+        env:
+          DO_LINT: true
+          DO_NO_STD: true
+          DO_DOCS: true
+        run: ./contrib/test.sh
+
+  Beta:
+    name: Test - beta toolchain
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v4
+      - name: Checkout Toolchain
+        uses: dtolnay/rust-toolchain@beta
+      - name: Running test script
+        env:
+          DO_NO_STD: true
+        run: ./contrib/test.sh
+
+  Nightly:
+    name: Test - nightly toolchain
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v4
+      - name: Checkout Toolchain
+        uses: dtolnay/rust-toolchain@nightly
+      - name: Running test script
+        env:
+          DO_FMT: false
+          DO_NO_STD: true
+          DO_DOCSRS: true
+        run: ./contrib/test.sh
+
+  MSRV:
+    name: Test - 1.56.1 toolchain
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v4
+      - name: Checkout Toolchain
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: "1.56.1"
+      - name: Running test script
+        env:
+          DO_NO_STD: true
+        run: ./contrib/test.sh