Improved coverage reporting; increased coverage for the parser

Update scoverage to 0.99.11, turn highlighting on, increase minimum coverage.

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 now no
reason it couldn’t be 100%, except time and sanity.

While I was at it, I discovered that we had no coverage for a large number of lines/branches in the AST/parser, so I added a scalacheck test to hit that code, and fixed some not-very-interesting problems it turned up.