update scoverage

Also:
- turn highlighting on
- increase minimum coverage

And excluded classes that we cannot test in non-integration tests:
- the REPL
- the MongoDB API wrapper and executor
- some RenderTree debugging code

This pretty much makes the coverage number meaningful; there’s no
reason it couldn’t be 100%.
Scalacheck test of unparsing-parsing round trip
fix some errors in un-parsing

- missing parens
- flattens not handled properly
- dis-ambiguate leading “distinct” proj.

Also a couple of minor issues in the RenderTree instance.
bump min coverage to 66
Merge pull request #431 from mossprescott/ready/parser-coverage

Improved coverage reporting; increased coverage for the parser