Allow static analysis of performance

[salman-ca]

Just a few general questions on liquid-rust.

1. would you say this implementation provides the same level of safety as the ruby version? i.e. since an end user can create their templates, they won't be able to do anything dangerous.

2. Is there a way to introspect the liquid document. I believe in the ruby version you can see how many if statements, loops etc. the template has (this is probably to analyze the general complexity of the template - or limit the # of certain tags).

Great work!

[epage]

1. I'd say this implementation is safer. The ruby version blurs the line between liquid and ruby and they have to go through extra hoops to ensure it is safe. The Rust version does not have any escape hatches like that.

2. As of right now, there is not a way to introspect the document. Currently, the document gets turned into Box<dyn SomeTrait> where the implementations of the trait aren't exposed such that you can't even dynamic cast them. I'd be interested to know what the use case would be for us to consider opening this up which also help us know what would be the best way to open it up.

[salman-ca]

Regarding #2, the use case is as follows. If you allow the end user to create their own templates, they might write very inefficient templates such as abuse of looping statements etc.
If you can introspect the types and # of statements, you could generate some kind of a usage score on the template (in addition to rendering the template - but introspection could prevent you from evening rendering if it is flagged as such).

That is the use case I am thinking about.

[epage]

So a static analysis profiler, interesting. I'll have to give this some thought on the best way to handle it.

On a related note, we also need to take care of #300.