Auto-complete function signatures - Part 1

@Benglin - I have split up PR: https://github.com/DynamoDS/Dynamo/pull/2688 into smaller PR's, of which this is the first. 
One other thing is, I I know these tests internally tested ```ParseStringToComplete``` method, but do we have unit tests written for ```ParseStringToComplete``` also? That can come as a separate PR, just want to know (because it seems like everything else is counting on it, it needs to be thoroughly tested somehow).