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. 
Yep, by the sound of "it would have blown up a long time ago", we know how seriously this could have impacted our users. So @monikaprabhu, please make sure you add these tests for the work, thanks!