src: track memory retainer fields
If retainers are embedded in retainers, direct tracking those lead to double tracking. Instead, use the base tracker to track the field objects.
cc @addaleax [ I should admit that I don't have full understanding of this feature, so let me know if this needs rewrite ]
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines