`Witness` node constructor does not use provided witness value for typing

When we construct a WitnessNode using `WitnessNode::witness` we can provide a witness. However, this constructor passes through to `types::Arrow::witness(NoWitness)`; i.e., it does not constrain the target type of the witness node based on the type of the witness data.

This allows the construction of broken programs even when pruning is otherwise done correctly.