fix internal meta-continuation comparison for continuation sharing

The check that the current meta-continuation matches the captured one
would always fail (I think), since the current meta-continuation is
pruned on capture. Keep a weak link to the original meta-continuation
to enable detection of capturing a continuation that matches or
extends one that was previously captured.

Enabling sharing exposed a problem with the code that saves
continuation marks for partial sharing, since that implementation
became out of sync with the main implementation (so merge the
implementations).