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

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines