Cache all lookups, added tests

[azidar]

Contributor Checklist

• [N/A] Did you add Scaladoc to every public function/method?
• [x] Did you add at least one test demonstrating the PR?
• [x] Did you delete any extraneous printlns/debugging code?
• [x] Did you specify the type of improvement?
• [N/A] Did you add appropriate documentation in docs/src?
• [x] Did you state the API impact?
• [x] Did you specify the code generation impact?
• [x] Did you request a desired merge strategy?
• [x] Did you add text to be included in the Release Notes for this change?

Type of Improvement

• bug fix

API Impact

No change to APIs, just fixes a bug where looking up the same value from the same Hierarchy[T] object returned a new object every time.

Release Notes

Bugfix: looking up the same value from the same Hierarchy[T] object now returns the same object, rather than a new object every time.

Reviewer Checklist (only modified by reviewer)

• [ ] Did you add the appropriate labels?
• [x] Did you mark the proper milestone (Bug fix: 3.4.x, [small] API extension: 3.5.x, API modification or big change: 3.6.0)?
• [x] Did you review?
• [x] Did you check whether all relevant Contributor checkboxes have been checked?
• [ ] Did you mark as Please Merge?