Add TypeArg::as_type_row helper for working with row variables.

[doug-q]

I needed them out of tree and wrote:

/// Fallibly convert a [TypeArg] to a [TypeRowRV].
///
/// This will fail if `arg` is of non-sequence kind (e.g. Type).
/// TODO move this to `impl TypeArg` in `hugr-core`
pub fn type_arg_into_type_row_rv(arg: impl Into<TypeArg>) -> Option<TypeRowRV> {
    match arg.into() {
        TypeArg::Sequence { elems } => elems
            .into_iter()
            .map(type_arg_into_type_rv)
            .collect::<Option<Vec<_>>>()
            .map(|x| x.into()),
        TypeArg::Variable { v } => {
            Some(vec![TypeRV::new_row_var_use(v.index(), v.bound_if_row_var()?)].into())
        }
        _ => None,
    }
}

/// Fallibly convert a [TypeArg] to a [TypeRow].
///
/// This will fail if `arg` is of non-sequence kind (e.g. Type)
/// or if the sequence contains row variables.
/// TODO move this to `impl TypeArg` in `hugr-core`
pub fn type_arg_into_type_row(arg: impl Into<TypeArg>) -> Option<TypeRow> {
    match arg.into() {
        TypeArg::Sequence { elems } => elems
            .into_iter()
            .map(type_arg_into_type)
            .collect::<Option<Vec<_>>>()
            .map(|x| x.into()),
        _ => None,
    }
}