Implement EagerIfTag

[jasmith-hs]

Part of #532

This PR adds the functionality necessary for eager execution with if tags. Unless tags are handled the same when executing eagerly as they are a negated if tag, and the branch paths are evaluated in the same way.

If the correct path cannot be determined because of a DeferredValueException, then the eagerInterpret() method gets called and all branches of the if tree get executed in a speculative fashion. The children are executed in a child context that is set to protectedMode = true. This protected mode prevents certain things from happening that would change the state of the context. When one of these occurs (such as replacing or updating a value in the interpreter.getContext()), the previous value gets reconstructed. See #545 as this logic is generic and has been introduced already.

Also, there is some logic to prune branches that certainly won't be executed. This is possible when there are ElseIfTags present so we don't need to include code unnecessarily.

---

A quick example:

{% set foo,bar = 'is foo','is bar' %}
{% if contact.registered %}
{{ foo }}
{% else %}
{{ bar }}
{% endif %}

This is a simple case, where eagerly executing the if statement (assuming contact is deferred) would cause the output to be:

{% if contact.registered %}
is foo
{% else %}
is bar
{% endif %}

A second pass with the contact set would either give is foo or is bar, without needing to preserve the context. This is even more beneficial when rather than evaluating simple expressions, complex functions (possibly calling APIs) can be eagerly evaluated within these branches.

cc @jboulter @Joeoh

[Joeoh]

This comment is a little unclear to me

[jasmith-hs]

Sorry, I have a typo in it. It's saying that if an elseif branch would definitely get executed, it can be changed to an else.
Ex.

{% if deferred %}
1.
{% elseif foo > 0 %}
2.
{% else %}
3.
{% endif %}

if foo == 2, then it could get output like:

{% if deferred %}
1.
{% else %}
2.
{% endif %}

[jasmith-hs]

And the opposite for definitelyDrop. If there were to be a tag like: {% elseif foo == 0 %}, it could get dropped as it's impossible.

[boulter]

if we drop a branch, what happens to the whitespace around it? I could see this changing the output of templates because of dropped statements and whitespace (not that it's generally useful).

[jasmith-hs]

This is a good question. The trimming gets handled when the tree gets parsed. Dropping a statement here is done similarly to picking the correct branch in the normal IfTag (and then not executing any of the others). Since no newlines are being added from the EagerIfTag's logic, the resulting whitespace is accurate.

The TreeParser takes the trim characters from a TagToken and moves them over to the TextToken children so we the whitespace trimming doesn't get handled in the tag node logic.