Implement First/Last

[gordonwatts]

Find the first or last item in a sequence. Bomb if there is nothing in the sequence!

• implement in the front end (new ast)
• Implement in the back end too!

Only need to do this for collections inside an event - not for all events.

[gordonwatts]

Now that we have the basics, we have to implement tuples and first.

(tuple).First()[index] => (tuple)[index].First() or similar, for example. You need a select in there, I suspect.