All Complex Specification Types Should be Reference-Like

[IgnoredAmbience]

Note this clarification text has been added to the spec regarding interpretation of variable bindings for record, list, and other complex specification types:

Algorithm steps may declare named aliases for any value using the form “Let x be someValue”. These aliases are reference-like in that both x and someValue refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want to avoid this reference-like behavior should explicitly make a copy of the right-hand side: “Let x be a copy of someValue” creates a shallow copy of someValue.

This was added in response to my clarification request in tc39/ecma262#752 noting that my interpretation of the behaviour of CompletePropertyDescriptor and its usage in [[GetOwnProperty]] for Proxy objects.

I imagine that to guarantee correctness and closeness to the spec in the future, we're going to need to adjust jsexplain's heap structure to match this.