Run code coverage on smart contracts

[sveitser]

This involves running forge coverage, probably with --report lcov and then uploading the file to coveralls.

      - name: Coveralls upload
        uses: coverallsapp/github-action@master
        with:
          github-token: ${{ secrets.GITHUB_TOKEN }}
          path-to-lcov: lcov.info

We probably don't want to run this on every PR but on pushes to main.