Cherry-pick ffbb4e8d542df to 26.2?

[Eric Abrahamsen]

I am still slowly grinding through updates to eieio-persistent, and
after having rushed changes into 26.1, I realized those changes were
incomplete, and made a further update to master, as ffbb4e8d542df. This
has been running for a while with no ill-effects, and I would like to
cherry-pick it to the 26.2 branch. Long story short, the
eieio-persistent write process is fully recursive, while the read
process is recursive for lists, but only goes one level deep for
hash-tables and vectors. This patch fully balances the read and write
processes.

In fact I'm still working on a rewrite of this whole thing in bug#29541,
but that's for master, this patch would at least make 26 correct.

Can I cherry-pick?

Eric