Support variants

[andry-dev]

The parser does not support sum-types (C++'s std::variants, Rust enums, ...), but it may be helpful to support them. How would a definition look like?

json Foo {
    // ...

    sum name {
        variant type="Bar" {
            key "a"
        }

        variant type="Baz" {
            key "b"
        }
    }
}

Ideas

• Calling it enum overloads the terms used for int-enum and str-enum, I would make it completely different (like the sum, which is closer to algebraic/type-theory).
• A sum-type variant must not contain optional #true. All elements must be equally possible.
• Nesting sum inside sum should not be possible (just make a new type).

[arves100]

Example in bf code: See reversed PermitInfo in UserInfoResponse