1290: Add Union to code_model r=matklad a=matklad
@flodiebold I am conflicted about two possible implementation approaches:
* we can add a separate `struct Union` to code model
* we can add `fn is_union(&self)` to existing `Struct`
This PR goes with the former approach, because it seems like Unions are sufficiently different in semantics to warrant a separate types. Which is in contrast to Syntax Tree, where both structs and unions share the same node kind, because their syntax is the same.
What would be the right thing to do here?
Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>
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