Clearer documentation on serialisable data

There’s a (relatively) new page on the devwiki, Serialisable Squirrel, which documents the exact restrictions on (say) what can be passed to agent.send() or http.jsonencode(), or stored in the nv table or in server.setpermanentvalues(). (Previously the wiki used to contain somewhat blithe and useless remarks such as saying that “almost all” data is serialisable.)

Anyway, we (and our beta-testers) have found several bugs in both our code and theirs due to making assumptions about serialisability – so, hopefully, a clearer statement of what’s supported will be helpful to us all…