add check for unused dependencies (bevyengine#2461)

# Objective

- Unused dependencies are bad

## Solution

- Add check for unused dependencies with https://github.com/est31/cargo-udeps - bevyengine#2455

.github/bors.toml

@@ -11,6 +11,7 @@ status = [
     "run-examples",
     "check-doc",
     "check-missing-examples-in-docs",
+    "check-unused-dependencies",
 ]
 
 use_squash_merge = true

.github/workflows/ci.yml

@@ -168,3 +168,22 @@ jobs:
           file-types: '[".rs"]'
           exclude-folders: '["./examples/ios"]'
           exclude-files: '[]'
+
+  check-unused-dependencies:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          override: true
+
+      - name: Installs cargo-udeps
+        run: cargo install cargo-udeps
+
+      - name: Install alsa and udev
+        run: sudo apt-get update; sudo apt-get install --no-install-recommends libasound2-dev libudev-dev
+
+      - name: Run cargo udeps
+        run: cargo udeps