Add optional 'safeToEncoding' method on 'SafeJSON'

[jkachmar]

I could open a PR for this, but I figured a discussion might be better first.

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Right now there's no way to use aeson's faster toEncoding JSON serialization methods (if they're defined) with safe-json.

As far as I can tell, it should be possible to add support for this with a new method on SafeJSON where a -> Contained Encoding. It could be optional (like aeson), with the default implementation calling safeTo encoding the resulting Value.

It gets a bit tricky with something like .=$, though. I imagine we would need to define a SafeKeyValue class so that .=$ could use safeToJSON on Pair and Object types, but safeToEncoding (or whatever it might be called) on Series types.

[Vlix]

I'm not against this addition, in principle.
As I understand it, the SafeKeyValue class wouldn't be something users of this library would have to define, right? It would just get instances of Pair, Object and Series?

If you'd like to make a PR, that's fine. We'll figure out the ergonomics when there's something tangible. I don't think it should be too difficult.
My suggestion would be to call the method safeEncoding and the function that uses that encoding and adds the version tag safeToEncoding.

(REMINDER: make Data.Aeson.Safe.encode and Data.Aeson.Safe.encodeFile use the new safeToEncoding)