Support aliasing macro function names in {% from %} tag

[jasmith-hs]

I just realised that this is something that I supported when implementing eager execution, but it was never supported in default execution. This is allowed in jinja, and we allow aliasing variables, so we should also allow aliasing macro functions:

From jinja docs:

Alternatively, you can import specific names from a template into the current namespace:

{% from 'forms.html' import input as input_field, textarea %}
<dl>
    <dt>Username</dt>
    <dd>{{ input_field('username') }}</dd>
    <dt>Password</dt>
    <dd>{{ input_field('password', type='password') }}</dd>
</dl>
<p>{{ textarea('comment') }}</p>