Analyze and document levelization:

* Markdown explanation of what levelization is, the intended levels, as
  well as the process used to determine dependencies
* Shell script finds all dependencies, groups them, and finds cyclic
  dependencies and maps out non-cyclic dependencies.
* Github job to run the script and fail if anything changes. Should
  catch introduction of new dependencies and new problems. Will also
  detect changes if problems or dependencies are removed.

.github/workflows/levelization.yml

@@ -0,0 +1,18 @@
+name: levelization
+
+on: [push, pull_request]
+
+jobs:
+  check:
+    runs-on: ubuntu-18.04
+    env:
+      CLANG_VERSION: 10
+    steps:
+    - uses: actions/checkout@v2
+    - name: Check levelization
+      run: Builds/levelization/levelization.sh
+    - name: Check for differences
+      run: git diff --exit-code
+      # If this workflow fails, and you have improved levelization, run
+      # Builds/levelization/levelization.sh, and commit the changes to
+      # loops.txt.