add a simple GitHub Actions CI workflow

Currently, this repo doesn't have any kind of CI set up to prevent us
from merging broken PRs. This has resulted in mistakes, at least on my
behalf (sorry). Therefore, this commit adds a quick CI setup for
ensuring that the code compiles and that it's been `rustfmt`ed
correctly.

We don't currently have any tests for the code in Idolatry, but if we
add some in the future, we'll want to add a job to run the tests on CI,
as well --- right now, it's just `cargo check` and `cargo fmt`.

.github/workflows/ci.yml

@@ -0,0 +1,65 @@
+name: CI
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+env:
+  # disable incremental compilation.
+  #
+  # incremental compilation is useful as part of an edit-build-test-edit cycle,
+  # as it lets the compiler avoid recompiling code that hasn't changed. however,
+  # on CI, we're not making small edits; we're almost always building the entire
+  # project from scratch. thus, incremental compilation on CI actually
+  # introduces *additional* overhead to support making future builds
+  # faster...but no future builds will ever occur in any given CI environment.
+  #
+  # see https://matklad.github.io/2021/09/04/fast-rust-builds.html#ci-workflow
+  # for details.
+  CARGO_INCREMENTAL: 0
+  # allow more retries for network requests in cargo (downloading crates) and
+  # rustup (installing toolchains). this should help to reduce flaky CI failures
+  # from transient network timeouts or other issues.
+  CARGO_NET_RETRY: 10
+  RUSTUP_MAX_RETRIES: 10
+
+jobs:
+  check:
+    name: Check
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+        - "stable"
+        # the current pinned Rust toolchain in the Hubris repo:
+        # https://github.com/oxidecomputer/hubris/blob/master/rust-toolchain.toml
+        #
+        # when updating Hubris' Rust toolchain, make sure to update this, too!
+        # TODO(eliza): it would be nice to determine this automatically...
+        - "nightly-2022-11-01"
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions-rs/toolchain@v1
+      with:
+        profile: minimal
+        toolchain: ${{ matrix.rust }}
+        override: true
+    - uses: Swatinem/rust-cache@v1
+    - run: cargo check --all
+
+  rustfmt:
+    name: rustfmt
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+        - "stable"
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions-rs/toolchain@v1
+      with:
+        profile: minimal
+        toolchain: ${{ matrix.rust }}
+        override: true
+    - uses: Swatinem/rust-cache@v1
+    - run: cargo fmt --check