tests/Runner.hs: add folds
Roundtrip pretty/parsing test
Add prettyparser golden files
ExactPrint: replace (most of) inexhaustive patterns with call to error
Eliminate inexhaustive patterns in InternalLexer
Eliminate inexhaustive patterns in ParseUtils
Eliminate inexhaustive patterns in Build
ParseSyntax: derive Functor
Eliminate inexhaustive patterns in ParseSyntax
Eliminate inexhaustive patterns in ExactPrint
Enable -fwarn-incomplete-patterns. Finally closes #94