FromJSON and ToJSON (aeson) instances for NvimObject

[saep]

I think this package transitively depends on aeson anyway, so it is probably convenient to provide such instances.

[saep]

The main point to think about here is that JSON does not allow non-string keys (IIRC) whereas messagepack does not have such a limitation. The string key limitation is also true for vimL, so an implementation would only have to support string keys unless I'm missing something.