Only use include_byte! on a file once across templates

[GuillaumeGomez]

Currently, everytime we encounter an include or an extends, we add an include_byte! call in the code. It's pretty bad and since we now keep things across the process, I think we should also keep this information to know if we actually need to generate another include_byte! call.

[GuillaumeGomez]

As discussed in #85, because of incremental compilation, it might ont have the desired behaviour. Eventually we could think about limiting duplicated include for a given type, but I'm not sure it's worth it at this point. Gonna leave it open for now in case I have some new idea.