decoder: Adjust Expression.ReferenceTargets() address arguments

[radeksimko]

As discovered while implementing this method for List

• decoder: Add support for List as Constraint #195

we will need to pass around an address which may not necessarily refer to the attribute as whole, but different expressions depending on levels of nesting.

This is expected to be useful for most other constraints which can have reference targets inside (e.g. Object or Map).

This also adds some compilation-time tests to ensure that we don't accidentally change interface somewhere and "turn off" collection of origins or targets.

[dbanck]

LGTM, while mildly confusing

We use address as a name for a lot of things: AttributeAddrSchema, lang.Address, schema.Address

[radeksimko]

We use address as a name for a lot of things: AttributeAddrSchema, lang.Address, schema.Address

Yeah, fair point. I think it was an act of balancing "easy to type" vs "easy to read" vs meaning. We could possibly call lang.Address => lang.ResolvedAddress and schema.Address => schema.AddressSchema, but I can't think of any simple way to get rid of the "address" in those.

For AttributeAddrSchema we could play with some combinations of target, or targetable, but nothing better comes up to mind right now.

All in all I agree it's not ideal, we can revisit later.