decoder: Finish reference collection implementation for Constraint

[radeksimko]

Context

For the following constraints:

• decoder: Add support for Set as Constraint #194
• decoder: Add support for List as Constraint #195
• decoder: Add support for Tuple as Constraint #198
• decoder: Add support for Map as Constraint #199
• decoder: Add support for Object as Constraint #203

we have implemented reference collection to our best effort/knowledge, but were unable to test most of that logic due to inability to use other "collectable" constraints, such as

• decoder: Add support for LiteralValue as Constraint #187
• decoder: Add support for LiteralType as Constraint #186
• decoder: Add support for Reference as Constraint #185

This is therefore a reminder to come back to all constraints and ensure that the implementation is indeed correct and tested properly.

Proposal

• reference origins: List - add tests
• reference targets: List - collect parent target for the whole list
• reference targets: List - add tests
• reference origins: Set - add tests
• reference targets: Set - collect parent target for the whole set
• reference targets: Set - add tests
• reference origins: Tuple - add tests
• reference targets: Tuple - collect parent target for the whole tuple
• reference targets: Tuple - add tests
• reference origins: Map - add tests
• reference targets: Map - add tests
• reference origins: Object - add tests
• reference targets: Object - add tests
• reference targets: LiteralType - add tests
• reference targets: LiteralValue - add tests