Add CI shell scripts

We would like to put all the CI scripts in a single place instead of
copied to each repository.

Add a `ci/` directory and in it a `run_task.sh` script as well as
auxilary scripts required. Include a README to document the directory.

ci/README.md

@@ -0,0 +1,216 @@
+# Continuous Integration
+
+This directory contains tools used by crates in the `rust-bitcoin` org to implement Continuous
+Integration. Currently this is just a script `run_task.sh` that can be called from a GitHub workflow
+job to run a specific task.
+
+TL;DR `./run_task.sh --help`
+
+#### Table Of Contents
+
+- [Usage](#usage)
+- [Lock file](#lock-file)
+- [Crates](#crates)
+  * [Per crate environment variables](#per-crate-environment-variables)
+  * [Additional crate specific tests](#additional-crate-specific-tests)
+- [Fuzzing](#fuzzing)
+- [Example workflows](#example-workflows)
+  * [A job using a stable toolchain](#a-job-using-a-stable-toolchain)
+  * [A job using a specific nightly toolchain](#a-job-using-a-specific-nightly-toolchain)
+
+## Usage
+
+The `run_task.sh` script expects a few things to be present when it runs:
+
+In the repository root:
+
+- A lock file `Cargo.lock`
+- A script that defines the crates: `contrib/crates.sh`
+
+And for each crate:
+
+- `test_vars.sh`
+- Optional: `extra_tests.sh`
+
+(See [Crates`](#crates) below.)
+
+## Lock file
+
+Repositories MUST contain a `Cargo.lock` file before running `run_task.sh`. `cargo` is typically
+called with `--locked`. If you don't care about dependency versions just run `cargo update` in your
+CI job (to create a lock file) before calling `run_task.sh`.
+
+If you do care about versions consider adding:
+
+- `Cargo-recent.lock`: A manifest with some recent versions numbers that pass CI.
+- `Cargo-minimal.lock`: A manifest with some minimal version numbers that pass CI.
+
+Then you can use, for example:
+
+```yaml
+    strategy:
+      matrix:
+        dep: [minimal, recent]
+    steps:
+
+    <!-- other stuff elided -->
+
+      - name: "Copy lock file"
+        run: cp Cargo-${{ matrix.dep }}.lock Cargo.lock
+
+```
+
+(Tip: Create minimal lock file with`cargo +nightly build -- -Z minimal-versions`.)
+
+## Crates
+
+All repositories MUST include a `contrib/crates.sh` script that declares the crates to be tested.
+
+This is a `bash` array (if repository is not a workspace just use `CRATES=(".")`).
+
+```bash
+#!/usr/bin/env bash
+
+# Crates in this workspace to test (note "fuzz" is only built not tested).
+CRATES=("base58" "bitcoin" "fuzz" "hashes" "internals" "io" "units")
+```
+
+### Per crate environment variables
+
+All crates MUST include a file `contrib/test_vars.sh`
+
+```bash
+#!/usr/bin/env bash
+
+# Test all these features with "std" enabled.
+#
+# Ignore this if crate does not have "std" feature.
+FEATURES_WITH_STD=""
+
+# Test all these features without "std" enabled.
+#
+# Use this even if crate does not have "std" feature.
+FEATURES_WITHOUT_STD=""
+
+# Run these examples.
+EXAMPLES=""
+```
+
+Tip: For non-worspace repositories this file should be in `contrib/` in the repository root.
+
+#### The `EXAMPLES` variable
+
+```bash
+EXAPMLES="example:feature"
+```
+
+```bash
+EXAPMLES="example:feature1,feature2"
+```
+
+```bash
+EXAPMLES="example_a:feature1,feature2 example_b:feature1"
+```
+
+
+Tip: if your example does not require any features consider using "default".
+
+```bash
+EXAPMLES="example_a:default"
+```
+
+### Additional crate specific tests
+
+Additional tests can be put in an optional `contrib/extra_tests.sh` script. This script will be run
+as part of the `stable`, `nightly`, and `msrv` jobs after running unit tests.
+
+As for other per-crate files, put it in either the `REPO/CRATE/contrib/` directory or
+`REPO_DIR/contrib/` directory depending on whether this is a workspace or not.
+
+## Fuzzing
+
+Fuzz tests are expected to be in a crate called `fuzz/`. The `run_task.sh` script just builds
+the fuzz crate as a sanity check.
+
+## Example workflows
+
+### A job using a stable toolchain
+
+To use the `run_task.sh` script you'll want to do something like this:
+
+
+```yaml
+jobs:
+  Stable:                       # 2 jobs, one per manifest.
+    name: Test - stable toolchain
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        dep: [minimal, recent]
+    steps:
+      - name: "Checkout repo"
+        uses: actions/checkout@v4
+      - name: "Checkout maintainer tools"
+        uses: actions/checkout@v4
+        with:
+          repository: rust-bitcoin/rust-bitcoin-maintainer-tools
+          path: maintainer-tools
+      - name: "Select toolchain"
+        uses: dtolnay/rust-toolchain@stable
+      - name: "Copy lock file"
+        run: cp Cargo-${{ matrix.dep }}.lock Cargo.lock
+      - name: "Run test script"
+        run: ./maintainer-tools/ci/run_task.sh stable
+```
+
+### A job using a specific nightly toolchain
+
+Have a file in the repository root with the nightly toolchain version to use.
+
+```bash
+$ cat nightly_version
+nightly-2024-04-30
+```
+
+And use a `Prepare` job to a set an environment variable using the file.
+
+```yaml
+jobs:
+  Prepare:
+    runs-on: ubuntu-latest
+    outputs:
+      nightly_version: ${{ steps.read_toolchain.outputs.nightly_version }}
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v4
+      - name: Read nightly version
+        id: read_toolchain
+        run: echo "nightly_version=$(cat nightly-version)" >> $GITHUB_OUTPUT
+
+  Nightly:                      # 2 jobs, one per manifest.
+    name: Test - nightly toolchain
+    needs: Prepare
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        dep: [minimal, recent]
+    steps:
+      - name: "Checkout repo"
+        uses: actions/checkout@v4
+      - name: "Checkout maintainer tools"
+        uses: actions/checkout@v4
+        with:
+          repository: tcharding/rust-bitcoin-maintainer-tools
+          ref: 05-02-ci
+          path: maintainer-tools
+      - name: "Select toolchain"
+        uses: dtolnay/rust-toolchain@v1
+        with:
+          toolchain: ${{ needs.Prepare.outputs.nightly_version }}
+      - name: "Copy lock file"
+        run: cp Cargo-${{ matrix.dep }}.lock Cargo.lock
+      - name: "Run test script"
+        run: ./maintainer-tools/ci/run_task.sh nightly
+```