Acknowledge the minimal functional backend, highlighting what's new/different for our SExpr targets.
Add and use the reference `minimal backend`.
Use `note` directives to point out missing code sections (highlighting that the included diffs are not complete).
Racket *is* still strongly-typed, it's just dynamic instead of static. Adjust phrasing to reflect that.
Adjust some of the literal includes, adding a new section from the `Functional::AbstractVisitor`, splitting the `Module::write_eval()` in twain and adding a `smtlib.cc` literal include for the node iteration, as well as for the `FunctionalSmtBackend` to compare against the minimal backend.
Move `Backend` description up to minimal functional backend section.
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