Generate code coverage information from CI

[daviddrysdale]

It would be nice to have a view of how much of the code of (particularly) the Runtime is covered when our tests are run.

• Possible external services: https://codecov.io, https://coveralls.io
  • Article of interest
• Tooling: https://github.com/mozilla/grcov

[daviddrysdale]

Hold open for the moment, to allow for:

• Improving the results:
  • reduce false negatives (lines that are red but seem like they must have been hit given the lines hit nearby)
  • some lines are showing yellow (implying branches missed) that don't seem to be branchable
• ~Adding a top-level badge / link to the results~. [Done in #1241]

[daviddrysdale]

cargo tarpaulin might be another alternative for generating coverage information, although it also has false negatives.

Note also #1886