Add a default “is true” selector.

Fixes #320.

This causes us to prefer `$project; $match` over a `$where` selector,
preventing us from falling into a map/reduce (as long as the condition
can be represented as a Mongo `$project` expression).

Also lets us filter on a constant … which is _maybe_ useful somehow?
Add an integration test for field comparisons.
Improve coverage

Remove dead code, add some WorkflowBuilder tests, and simplify a couple
pattern match cases.

There’s more to be done (of course), but hopefully this will put us back
above the minimum. Also, the re-colored scoverage output is super helpful.
Merge pull request #464 from sellout/ready/improve-coverage

Improve coverage
Merge pull request #462 from sellout/ready/320-comparisons-between-fields

Add a default “is true” selector.
simple test
Merge remote-tracking branch 'upstream/master' into wip/443