Add BoxCodec

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FsCodec.Box.Codec is a drop-in-equivalent for FsCodec.NewtonsoftJson.Codec with equivalent .Create overloads which encode as ITimelineEvent<obj> (as opposed to ITimelineEvent<byte[]>.

This is useful when storing events in a MemoryStore as it allows one to take the perf cost and ancillary yak shaving induced by round-tripping arbitrary event payloads out of the picture when writing property based tests.

NOTE this does not imply one should avoid testing this aspect; the opposite in fact -- one should apply the Test Pyramid principles:

• have a focused series of tests which validate that the various data represented in the event bodies are round-trippable a) in the chosen encoding format (i.e. UTF8 json) b) with the selected concrete json encoder (i.e. json.net for now 🙁)
• integration tests can in general can use BoxEncoder and MemoryStore
• you should of course have acceptance tests that apply the actual serialization encoding with the real store for a representative number of scenarios at the top of the pyramid