Trailing trivia

These changes move the existing fidelity syntax model closer to the new AST model.  Specifically, it moves away from having both leading and trailing trivia on tokens (like Roslyn) and only has leading trivia.  Also, it simplifies how skipped tokens are handled when the parser encounters tokens it cannot handle.

The hope here is that by making Fidelity more like the new system, we can more easily add incremental parsing to the new system as the Fidelity algorithms will more closely match.