Ruby <2.7now uses WeakMap too, which prevents memory leaks.

Ruby <2.7 does not allow non-finalizable objects to be WeakMap
keys: https://bugs.ruby-lang.org/issues/16035

We work around this by using a secondary map for Ruby <2.7 which
maps the non-finalizable integer to a distinct object.

For now we accept that the entries in the secondary map wil never
be collected.  If this becomes a problem we can perform a GC pass
every so often that looks at the contents of the object cache to
decide what can be deleted from the secondary map.