Fix memory accounting to detect when messages pile up in a
place channel and when shared values (such as the result of
`make-shared-bytes') pile up. Also fix problems where a GC
or free-page purge needs to be triggered.
The implementation causes a minor API change, which is that
a place channel sent multiple times as a message generates
values that are `equal?' but no longer eq?'.
Closes PR 12273
[Do not merge to 5.2]
5c0956d7b1
