CiviCRM Webform : Select field used to use autofill
This is not exactly what you describe but may suit your purposes or at least give you something to go on.


In the CiviCRM tab of the webform builder, enable CiviCRM processing. For Contact 1, tick Existing Contact and a few other fields (try including some address fields for example). 
Go to the Webform tab and click Edit next to the Existing Contact component. Set it to Autocomplete and then in the Contact Display Fields box, use Cntrl-Click to add a bunch of fields that match the ones you picked on the CiviCRM tab.


Go to View and now you should find that it works kind of like you describe. Start typing in the box and contacts should appear with First Name, Last name and email all in the box and when you pick one the other fields should populate.