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.
Introduced expanders for events (define-event-expander) and for
assertion-patterns (define-assertion-expander).
Introduced convenience syntax and utilities in web.rkt for working
with web requests.
Support nested bindings in assertion-patterns for message events
(only).
Tony Garnock-Jones