Lazy look_ahead

[audunhalland]

I've noticed that when using the look-ahead feature, this always involves a full traverse/transform of the input document under that node. This can be a problem when the input documents are large and complex.

This is an attempt to rewrite the look_ahead API so that its invocation becomes basically free. Instead of doing a forced deep transformation up front, the required work has been moved into the methods explicitly called by the API user.

A single level of LookAheadChildren gets computed when the user calls LookAheadSelection::children(). The arguments/values to a field are also lazily computed, and variable substitution is now done purely on demand.

The PR is in a draft state and the old look_ahead API is still intact for the time being, I'm interested in general feedback first before finishing it off.

Summary of API breakages:

• trait LookAheadMethods removed. In my opinion there's no need for this and it always requires an explicit extra import.
• struct ConcreteLookAheadSelection removed
• struct LookAheadArguments removed, replaced by an opaque impl Iterator
• struct LookAheadSelection now has some inherent methods at least partially compatible with LookAheadMethods:
  • field_name, has_arguments: Compatible signature
  • arguments now returns impl Iterator<Item = LookAheadArgument> instead of &[LookAheadArgument].
  • argument now returns Option<LookAheadArgument> instead of Option<&LookAheadArgument>.
  • children now returns LookAheadChildren instead of Vec<&Self>. LookAheadChildren implements IntoIterator.
  • field_original_name / field_alias: New in 0.16
• Some methods moved into new struct LookAheadChildren:
  • LookAheadMethods::child_names
  • LookAheadMethods::has_children moved to LookAheadChildren::is_empty
  • LookAheadMethods::select_child moved to LookAheadChildren::select
• LookAheadSelection::for_explicit_type moved to LookAheadSelection::children_for_explicit_type, which returns LookAheadChildren.
• LookAheadValue::List(_) contains a new struct LookAheadList with IntoIterator + iter method.
• LookAheadValue::Object(_) contains a new struct LookAheadObject with IntoIterator + iter method.

[audunhalland]

I know the change looks large, I made it so because I essentially see it as a rewrite. I would like to get some indication on whether you would accept a change along these lines. If you are positive, I'll finish up the PR, overwrite the existing implementation and update the integration tests. I've finished this locally.

[tyranron]

@audunhalland I'll take a look in a week or so.

[audunhalland]

@tyranron Thanks! If you instead prefer a PR closer to the finished product, I can fix that before that time.

[tyranron]

@audunhalland once you finish adjustments, please, re-request my review via this GitHub button. Thanks!

[audunhalland]

Ok, rebased on master, overwrote look_ahead and updated the tests I found to be relevant (based on grepping for look_ahead).

edit: I think the changelog should also be updated, maybe just use the change summary in the PR description?