Unsafe unwrap in constant substitution

I don't think calling unwrap is safe here. It's quite easy to create ergo tree that references nonexistent constant

https://github.com/ergoplatform/sigma-rust/blob/3e09084d7acbe1cabd7424e5c31c69a1bf09ffa1/ergotree-ir/src/serialization/expr.rs#L92-L96

Also why constant substitution is even needed? It breaks roundtrip property. If constants are substituted it's no longer possible to serialize script back.