This operation effectively allows sending an expression back to a
continuation, instead of just a value. It's the same as Marc Feeley's
`continuation-slice` operation, but adjusted slightly to support
continuation attachments.
original commit: d0e36e72d20a6eaa5d9d8b795da5e77abde75289
Also, rename `call-with-current-continuation-attachment` to
`call-getting-continuation-attachment`.
original commit: e2a00e6d641b92918c4911c27ba14949748fd291
