Cache all lookups, added tests

[azidar]

Contributor Checklist

• [N/A] Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• [N/A] Did you add appropriate documentation in docs/src?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• 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?
• 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)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?

[jackkoenig]

I'm not sure if I can construct the case where it will matter, but my intuition is still that this should be an IdentityHashMap