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).
Make #lang syndicate module-begin gather boot actions into a
syndicate-main submodule, and for each such module, add a main
submodule that calls run-ground with the syndicate-main boot actions.
This lets us write syndicate *libraries* that comprise both
data-structures, functions, and Syndicate services.
Add support for encoding prefab structs as JSON objects.
Remove linkage & at-meta from patches inside broker dataspace. This is
ugly, and deserves to be revisited in future.
Fix a bug where using trie-prune-branch was incorrect, and
trie-subtract should have been used instead.
Factor out support/struct.rkt.