StructDef::fields and StructDef::set_fields don't take visibility into account

[asterite]

Aim

In #6221 we'll introduce struct field visibility. The existing macro methods for dealing with struct fields don't know about visibility, but they probably should.

Expected Behavior

There should be a way to know a struct field's visibility, and to specify it when using set_fields.

Bug

There's no way to deal with struct field visibilities in macros.

To Reproduce

Workaround

None

Workaround Description

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Additional Context

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Project Impact

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Blocker Context

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Nargo Version

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NoirJS Version

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Proving Backend Tooling & Version

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