This is a major change to the previous design, and also a change with
respect to the semantics in the ESOP 2016 paper. All the complexity of
echo-cancellation is stripped out of the core dataspace semantics, and
the relaying protocol is changed from one constructor, `at-meta`, to
two, `inbound` and `outbound`. The relay connecting a dataspace to its
container is now completely symmetric with the contained actors: it
initially asserts interest in what it is to relay, just like any other
actor would. Dataspaces no longer treat relaying specially.
This commit has updated all (I think) of the non-graphical examples. The
graphical code remains to be done in a following commit.
Prior to this commit, the stop-whens were using
current-inexact-milliseconds in a comparison to detect a timeout,
meaning that timeouts would be missed. This commit introduces a
redundant copy of current-inexact-milliseconds in a field and changes it
on every event so that stop-when expressions involving the field are
reevaluated properly frequently.