add CI

.github/bors.toml

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+delete_merged_branches = true
+status = ["continuous-integration/travis-ci/push"]

.travis.yml

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+language: rust
+
+matrix:
+  include:
+    - env: TARGET=x86_64-unknown-linux-gnu
+      rust: nightly
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
+
+    - env: TARGET=thumbv7m-none-eabi
+      rust: nightly
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
+
+before_install: set -e
+
+install:
+  - bash ci/install.sh
+  - export PATH="$PATH:$PWD/gcc/bin:$PWD/qemu"
+
+script:
+  - bash ci/script.sh
+
+after_script: set +e
+
+cache: cache
+
+before_cache:
+  - chmod -R a+r $HOME/.cargo;
+
+branches:
+  only:
+    - master
+    - staging
+    - trying
+
+notifications:
+  email:
+    on_success: never

ci/install.sh

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+set -euxo pipefail
+
+main() {
+    if [ $TARGET != x86_64-unknown-linux-gnu ]; then
+        rustup target add $TARGET
+    fi
+}
+
+main

ci/script.sh

@@ -0,0 +1,7 @@
+set -euxo pipefail
+
+main() {
+    cargo check --target $TARGET
+}
+
+main