Add a basic lookahead object

[rmosolgo]

I'm trying to migrate from irep_node:-based lookahead, so I want to implement something that will work with the current execution code and with the future execution code, i think this will work.

[rmosolgo]

Oops, this won't work in cases like

{
  f1 { a } 
  f1 { b } 
}

because it only keeps the first AST node matching a selection. So selection("f1").selection("b").selected? will be false, when it should be true.

Just like execution, it should keep a list of AST nodes, not just 1.

[rmosolgo]

This still doesn't work great with Unions, I mean, fields that return unions won't play nice with lookahead yet.

[rmosolgo]

I documented that shortcoming, we can cross that bridge when we get there.

[ianks]

@rmosolgo is it possible to get all selected fields with this API?

[rmosolgo]

Not yet, I put it off because I didn't need it yet. I think a nice API would be to add #selections which returns a list of Lookaheads for each selection, and then make each Lookahead respond to #name, so that you can tell what it represents.

next_lookaheads = lookahead.selections  
# => [#<GraphQL::Execution::Lookahead ...>, ...]
next_lookaheads.map(&:name) 
# => ["next", "field", "names", "go", "here"]

What do you think? Want to take a crack at it?