Merge skolem and mono flags in the variable descriptor

[jstolarek]

We might consider merging skolem and monormophic flags of the variable descriptor in the unifier. This flag would have one of those values:

• Skolem: skolem, therefore it doesn't make sense to track monomorphism
• Monomorphic
• Unconstrained: "flexible", non-monomorphic variable

This would exclude the case that monomorphic and skolem are true at the same time, which is an invalid state.