Add GHA CI job to run build, fmt, clippy, and test

This commit adds a CI job to github actions that will run pre and post
merge on any branch to run cargo build, cargo fmt, cargo clippy, and
cargo test. This will ensure that any proposed change to rustiq-core
will pass all of these checks to ensure the repo can always build, pass
all the tests, and is following standard best practices for rust
development.

We shouldn't add this CI job until #2 merges because CI won't pass
clippy without that.

.github/workflows/main.yml

@@ -0,0 +1,27 @@
+---
+name: Cargo Build & Test
+
+on:
+  push:
+  pull_request:
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  build_and_test:
+    name: Rust project - latest
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        toolchain:
+          - stable
+          - beta
+          - nightly
+    steps:
+      - uses: actions/checkout@v4
+      - run: rustup update ${{ matrix.toolchain }} && rustup default ${{ matrix.toolchain }}
+      - run: cargo build --verbose
+      - run: cargo fmt --all -- --check
+      - run: cargo clippy --all-targets -- -D warnings
+      - run: cargo test --verbose