Implement delta-observational equivalence (#244, #299)
Implement delta-observational equivalence (#244, #299)

This proof shows that delta-observational equivalence is an equivalence
relation.

Known limitations of this commit:
* it does not show that any function respects this relation;
* it does not rephrase existing results;
* this might better belong in Base.Change.Equivalence.

Old-commit-hash: 46789e08b7a8bbd95738f9bf14b3ff8644151772

inc-lc/ilc-agda