[v2] Extend AST to wrap terminals

[ggiraldez]

Spawned from this comment thread.

Extends the IR/AST node-type model so the codegen produces a richer, more useful tree.

• Avoid re-exporting IR types in the AST. This will make it easier to evolve them independently (eg. the AST will need to be serializable to JSON).

• Unique terminals now have a real IR struct (Rc<XStruct> with id: NodeId and range). Optional sequence fields go from bool to Option<X>, choice variants carry an (X) payload, and the AST wraps them.

• Non-unique terminals are now wrapped in the AST as well. This will allow implementing some methods on them that require access to semantic information (eg. getting the type associated to a string expression).

• Avoid re-using some IR types for typing/binding information. Eg. function visibility and mutability are now plain variant defined in types. We can no longer easily re-use the IR types (reason being the previous item).

• NodeType::Nonterminal is gone and the IR-codegen model now distinguishes Sequence, Choice, Collection, Terminal, UniqueTerminal, and the new External kind. Templates dispatch on kind/is_terminal/is_external directly instead of looking up the kind on the side. Choice variants for unique terminals lose their Eq/Hash derives along the way (those types no longer make sense as set keys).

• Shorten the auto-generated expression_<X>_expression_operator precedence-operator field names down to a uniform operator everywhere.

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