Function unify 

pub fn unify(
    a: &Type,
    b: &Type,
    subst: &mut Substitution,
) -> Result<(), TypeError>

Unify two types under the given substitution, extending the substitution in place when a type variable is bound.

Follows standard Hindley-Milner rules:

• Type::Error unifies with anything (poison — prevents cascading errors).
• Type variables are bound after an occurs check.
• Structural types are unified component-wise.

Errors

Returns a TypeError if the types are not unifiable.