Give an :id to every member of the stack

[vemv]

Brief

Fixes #38

...It uses an alternative approach to that described in #38: Instead of a supdate-based map, we leverage the fact that all members of the stack are data (arrays of maps).

So we can just associate an :id, and let users customize formatting-stack by writing Clojure however they wish.

Notes

The :id requirement (w/ spec) is a breaking change, because a user that provided a map-based custom member would have to add an :id to have things working again.

Users passing a reify will see no :id requirement (there's a comment in the diff explaining that).

QA plan

Run the provided example, observing that the provided customizations are honored.

Author checklist

• I have QAed the functionality
• The PR has a reasonably reviewable size and a meaningful commit history
• I have run the branch formatter and observed no new/significative warnings
• The build passes
• I have self-reviewed the PR prior to assignment
• Additionally, I have code-reviewed iteratively the PR considering the following aspects in isolation:
  • Correctness
  • Robustness (red paths, failure handling etc)
  • Modular design
    • The examples could be moved to a examples/ folder, in a future, if they keep growing.
  • Test coverage
  • Spec coverage
  • Documentation
  • Security
  • Performance
  • Breaking API changes
  • Cross-compatibility (Clojure/ClojureScript)

Reviewer checklist

• I have checked out this branch and reviewed it locally, running it
• I have QAed the functionality
• I have reviewed the PR
• Additionally, I have code-reviewed iteratively the PR considering the following aspects in isolation:
  • Correctness
  • Robustness (red paths, failure handling etc)
  • Modular design
  • Test coverage
  • Spec coverage
  • Documentation
  • Security
  • Performance
  • Breaking API changes
  • Cross-compatibility (Clojure/ClojureScript)

[thumbnail]

Seems trivial enough 👍