Expand samples / templates / reference apps

Overhang work from jet/equinox#55 and jet/equinox#56:
- [x] (I'm likely to work on this next as a backburner task) Add TodoBackend sample (think: https://github.com/panesofglass/TodoBackendFSharp, but on aspdotnetcore like https://github.com/ChristianAlexander/dotnetcore-todo-webapi/blob/master/src/TodoWebApi/Controllers/TodosController.cs so we can run it via https://www.todobackend.com/) - see jet/equinox#63
- [x] Merge `feature/inventory` branch ? (I've put an `InventoryItem.fs` on `master` which is a port of https://github.com/gregoryyoung/m-r/tree/master/SimpleCQRS). The [associated GUI](https://github.com/gregoryyoung/m-r/tree/master/CQRSGui) is showing its age and to be honest I don't see much point progressing in that direction given how it's kinda a contrived tech example.
- [x] allows us to e.g. do a sample of serialization using Fleece instead of json.net, without loading that dependency, volume of code and conceptual complexity onto this repo (moved to https://github.com/jet/FsCodec/issues/6)
- [x] apply CQRS to Todo app - see #29
- [ ] Polish Web to make it more representative? i.e. `Cart` does not have a controller, and there is no checkout. I don't personally have much interest or bandwidth to do it at present, although I believe there's a good opportunity to do a very nice minimal example of event sourcing which most people can relate to
- [ ] Work with @mateusz-kula to do a simplified edition of Store, TodoBackend, which would be emitted from a generator to add:
- a GUI clickable way of running something on Equinox (i.e. the Web endpoint only has APIs; we can give instructions for postman and curl all we like, it won't feel the same to some folks)
- consumer driven contracts (stuff used in the genned app will be considered to be core API, even before we start aggressively versioning)
- allows us to cut corners for pedagogical reasons
- (i.e. SFL and Favorites both do some very fancy pants use of .NET non mutable data structures for perf reasons which are interesting and important, but would be better replaced with more compact immutable representations)
- can also demonstrate testing story as more direct example based tests