Add eager deferred macro logic

[jasmith-hs]

Part of #532

This PR unstubs the getNewlyDeferredFunctionImages() method in the EagerTagDecorator class by introducing an EagerMacroFunction wrapper class, which wraps the logic of a MacroFunction to be able to reconstruct a partially evaluated image of the macro function to output before a deferred call of the said macro function.

Since macro functions can either be called directly (when they are stored as a global macro function), or with an imported alias, such as {% import path as simple %} {{ simple.the_macro() }}, there needs to be a way to create a macro in this way. So I formalised this aliased macro importing by calling it a local macro, rather than a global macro (because it's only local to the alias). The MacroTag now allows for a macro function to be stored in such a way: {% macro parent_alias.macro_name() %}, which allows for global or local macro functions on the context to be re-output before they're called.

---

For example:

{% macro plus_one(val) %}
{{ val + 1 }}
{% endmacro %}
{% import import_path as my_alias %}
{{ plus_one(deferred.age) }}
{% if my_alias.can_vote(deferred.age) %}
You can vote.
{% endif %}

Would result in an output similar to the following if doing eager execution (adjusted for readability):

{% macro plus_one(val) %}
{{ val + 1 }}
{% endmacro %}
{{ plus_one(deferred.age) }}
{% set my_alias = {} %}
{% macro my_alias.can_vote(val) %}
{{ val >= 18 }}
{% endmacro %}
{% if my_alias.can_vote(deferred.age) %}
You can vote.
{% endif %}

cc @boulter @Joeoh