Node identity should depend on inputs, not nsspec

[Zaharid]

Right now two nodes from provider functions are considered equal if their namespace matches. The equality should instead explicitly consider the inouts of the nodes (that by assumption steam from the user's config ultimately). This would avoid duplicating computations that are known to have the same result.

One problem is that (nested) YAML lists and mappings can be used as inputs to the providers, and we would need a way to freeze them to make them hashable.