Add system for evaluating coverage of created assertions

[selkamand]

see #26 for context.

We should build

1. utilities for extracting the names of all assertion functions in our codebase
2. utilities for counting the number of unit tests (i.e. expect_ statements that test each assertion)
3. a system that produces a badge describing (proportion of assertions with > some_threshold unit tests)

Some potential options for badges here:
basically we just need to produce a json file somewhere in this repo that we could point to.

https://css-tricks.com/adding-custom-github-badges-to-your-repo/

[selkamand]

For now I think we ditch the badge idea.

Instead, I've added a unit test that only passes if all assert_ functions have at least 5 unit tests (expect statements).

Caveats:

1. assert_create and assert_create_chain are not included, but already quite well covered
2. detection of unit tests is a little rough. It only detects them if the expect statement and assert_ function are in the same line

Now the only way to pass unit tests is to have well tested code. Unfortunately its currently based on # of unit tests rather than code coverage.

Longer term, if covr or other solutions become able to calculate coverage of functions created upon loading of a package, that would supplement this solution nicely