[Enhancement] Cache garbage collectible variables for each file

[jonobr1]

Describe the solution you'd like
In this PR I introduced the pattern to have a cache object at the top of Group.js. I'd like to do this to every file to cut down on any garbage collection that happens on "per method invocations".

[jonobr1]

Audit note before expanding this pattern: the existing module-level cache in Group.getShapesAtPoint already leaks observable mutable state. Two successive calls with mode set to deepest return the same array object; retaining the first result and making a second query changes the first result in place. The cache is also vulnerable to re-entrant calls from the filter callback. Any broader caching work should require non-aliased public return values and re-entrancy tests.