Add a rust build cache for workflows (#17)

Adds two github cache actions. One for the toolchain and one for the
downloaded and built dependencies

.github/workflows/rust.yml

@@ -20,6 +20,22 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
+    # just triggers rustup to download the toolchain
+    - name: Cache Toolchain
+      uses: actions/cache@v3
+      with:
+        path: ~/.rustup
+        key: ${{ runner.os }}-rust-toolchain-${{ hashFiles('rust-toolchain.toml') }}
+    - name: Cache Dependencies
+      uses: actions/cache@v3
+      with:
+        path: |
+          ~/.cargo/bin/
+          ~/.cargo/registry/index/
+          ~/.cargo/registry/cache/
+          ~/.cargo/git/db/
+          target/
+        key: ${{ runner.os }}-rust-deps-${{ hashFiles('Cargo.lock', 'rust-toolchain.toml') }}
     - name: Build
       run: cargo build --verbose
     - name: Run tests