Cross platform builds in CI (#133)

This PR adds a GH workflow that builds across platforms, specifically the android platforms referenced in the cross_build.sh scripts, but can be extended to build for others. It seems like one of the next steps is to hook up version publishing in general for these packages/platforms, but this PR just focuses on getting the builds running in CI for continuous validation/testing and further iteration. The builds are still relatively slow in CI, taking about 15 minutes to complete, but not nearly as slow as when running locally on a mac via qemu.

https://github.com/bitcoindevkit/bdk-ffi/blob/master/.github/workflows/publish-android.yaml seems like a good example of the full build + publish workflow that we'd want to evolve this into.

Note that the `x86_64-linux-android` build is failing, with some context in #134. I spent some time trying to figure this out, unsuccessfully, and ultimately decided to spin up an issue to track it separately and get a PR up for the builds that work.

.github/workflows/build.yml

@@ -0,0 +1,53 @@
+name: Build
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+jobs:
+  build:
+    name: build
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        target: [
+            aarch64-unknown-linux-gnu,
+            # x86_64-linux-android,
+            i686-linux-android,
+            armv7-linux-androideabi,
+            aarch64-linux-android,
+          ]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v1
+
+      - name: "Cache"
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            ./target/${{ matrix.target }}
+          key: ${{ matrix.target }}-${{ hashFiles('**/Cargo.toml','**/Cargo.lock') }}
+
+      - name: Install rust
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          profile: minimal
+          override: true
+          target: ${{ matrix.target }}
+
+      # Install latest cross to mitigate unwind linking issue on android builds.
+      # See https://github.com/cross-rs/cross/issues/1222
+      - name: Install rust cross
+        run: |
+          cargo install cross --git https://github.com/cross-rs/cross
+
+      - name: Build target
+        uses: actions-rs/cargo@v1
+        with:
+          use-cross: true
+          command: build
+          args: --release --target ${{ matrix.target }} --manifest-path bindings_ffi/Cargo.toml --target-dir bindings_ffi/target

xmtp/Cargo.toml

@@ -45,4 +45,4 @@ tempfile = "3.5.0"
 
 [dependencies.libsqlite3-sys]
 version = "0.26.0"
-features = ["bundled-sqlcipher"]
+features = ["bundled-sqlcipher-vendored-openssl"]