Simple Code Coverage analysis (#378)

Closes #350.

Adds simple code coverage analysis. Until the GitHub token is set up in this repo, you can see the results including a comment on the PR here: mattpolzin#2

* Adding codecov CI workflow.

* doh. forgot to skip build.

* drop unneeded llvm env var from build invocation. make comment always run.

* try a way to update a previous comment

* try to get comments to run on failure.

* use bug fix to codecov action.

* fix comment search string and drop min coverage

* use replace on comment updates

* attempt at diffing coverage from main branch

* comment out tests to affect test coverage

* try method for multiline outputs

* do i need to put it in its own script file

* right, switch coverage gen order to have branch checked out second.

* attempt to get code quotation around diff output.

* switch to git diff

* add note about the new script.

* try echoing a warning.

* post warning on success and error on failure.

* uncomment tests

.github/workflows/codecov.yml

@@ -0,0 +1,35 @@
+
+name: Codecov
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  codecov:
+    container:
+      image: swift:5.3.2-bionic
+    runs-on: ubuntu-latest
+    steps:
+    - run: apt-get update && apt-get install -y gtk+-3.0 libgtk+-3.0
+    - name: Checkout Branch
+      uses: actions/checkout@v2
+    - name: Build Test Target
+      run: swift build --enable-test-discovery -Xswiftc -profile-coverage-mapping -Xswiftc -profile-generate --product TokamakPackageTests
+    - name: Run Tests
+      run: swift test --enable-test-discovery --enable-code-coverage --skip-build
+    - name: Generate Branch Coverage Report
+      uses: mattpolzin/swift-codecov-action@0.6.1
+      id: cov
+      with:
+        MINIMUM_COVERAGE: 20
+    - name: Post Positive Results
+      if: ${{ success() }}
+      run: |
+        echo "::warning file=Package.swift,line=1,col=1::The current code coverage percentage is passing with ${{ steps.cov.outputs.codecov }} (minimum allowed: ${{ steps.cov.outputs.minimum_coverage }}%)."
+    - name: Post Negative Results
+      if: ${{ failure() }}
+      run: |
+        echo "::error file=Package.swift,line=1,col=1::The current code coverage percentage is failing with ${{ steps.cov.outputs.codecov }} (minimum allowed: ${{ steps.cov.outputs.minimum_coverage }}%)."
+