try adding FunctionTypeVarArgs

[doug-q]

A bunch of renames should happen, which I have omitted to reduce the diff. In order:

• rename FunctionType::new to new_unchecked
• make the members of FunctionType private
• Invert the newtyping of FunctionType and FunctionTypeVarArgs
• rename FunctionType to Signature
• rename FunctionTypeVarArgs to FunctionType

[github-actions]

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