Pass expander whenever we need to expand something

[rgrinberg]

Rather than passing separate arguments to pass to the expander.

There's still a few more cases to handle. I will do those once #1554 is merged

[rgrinberg]

This is now ready. Now whenever we want to expand variables in the code we need to have an Expander.t instead of a bunch of related stuff. There's still a bit of tension for how to best obtain this expander. There's a couple of options so far:

• Get it from the dir and Super_context. Requires the least boilerplate whenever those are already there.

• Pass the expander manually.

I've opted mostly for the latter because it's similar to how we handle the scope. There's a few cases where I had to fallback to the former as things like the Compilation context and sub system context don't have expanders yet.

[rgrinberg]

Btw, along with the upcoming rule_context. Here's a summary of the kind of values we'll have to construct rules:

• Rule_context - whenever we need to add rules
• Expander - whenever we evaluate OSL/templates/blang
• Scope - whenever we interact with the library database
• Compilation context - whenever we compile OCaml code

This should make things a bit more clean in our code.