meta(ci): improve CI workflow

.github/workflows/rust.yml

@@ -2,12 +2,33 @@ name: Rust
 
 on: [push, pull_request]
 
+env:
+  RUSTFLAGS: -Dwarnings
+
 jobs:
-  build:
+  fmt_and_docs:
+    name: Check fmt & build docs
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: Install Rust
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          components: rustfmt
+          override: true
+      - name: rustfmt
+        run: cargo fmt --all -- --check
+      - name: docs
+        run: cargo doc
+
+  build_and_test:
+    name: Build & Test
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        rust: [stable, beta, nightly]
+        rust: [1.39.0, stable]
         os: [ubuntu-latest, macOS-latest, windows-latest]
 
     steps:
@@ -17,12 +38,10 @@ jobs:
         with:
           profile: minimal
           toolchain: ${{ matrix.rust }}
-          components: rustfmt, clippy
+          components: clippy
           override: true
       - name: Check
         run: cargo check
-      - name: Rustfmt
-        run: cargo fmt --all -- --check
       - name: Clippy
         run: cargo clippy -- -D warnings
       - name: Run tests