Reduce copies of S7 class objects - #743

I benchmarked this locally against main (R 4.5.3, aarch64 macOS; bench 1.1.4, lobstr 1.2.1). I ran bench/constructor.R twice in alternating order and averaged the two reported medians. I also corrected the memory case locally so it creates one class and passes two instances of that class to lobstr::obj_sizes(); previously it measured two separately created class definitions.

Marginal retained memory per instance:

So the PR achieves the intended result: per-instance class storage becomes a flat 168 B rather than growing with the class hierarchy.

Construction timings (average of the two median runs):

validate5 was effectively unchanged (+1%). The timing cost is most visible for lightweight/deep constructors, where the extra class-reference lookup is a larger share of total work; it disappears as property work dominates.

Follow-up class-operation benchmarks, now in a separate classes benchmark section (depth-5 class; same local setup as above):

This confirms the expected downstream benefit of shared class identity: operations whose first step is an identity comparison become roughly 24–35× faster. Direct class retrieval is slightly slower because the C accessor must dereference the environment-backed class reference.

I also prototyped having S7-generated constructors pass the class reference explicitly to new_object(), bypassing the dynamic get0() lookup. I ran three alternating baseline/prototype sessions and averaged the reported medians:

So explicit passing saves about 0.6 µs per new_object() call, consistent with the isolated lookup benchmark, but it does not materially move whole-constructor performance (~2%, below our 10% threshold). I reverted the prototype rather than complicating generated calls and the new_object() interface.

I set hash = FALSE on both new.env() calls introduced by this branch and benchmarked the change back-to-back.

• Class definition: 73 µs → 72 µs, effectively unchanged.
• Allocation per class definition: 8.41 KB → 7.86 KB, about 1.07x less.
• Constructor, class lookup, and instance memory benchmarks were unchanged within noise; all differences were ≤3%.

Focused class and constructor tests pass: 163 assertions.