Abstract ByteString out of MIME rendering

[dmjio]

This should allow us to abstract over ByteString from the MimeRender / MimeUnrender classes, without breaking any code and preserving existing semantics, while gaining extensibility to other platforms.

On the client (e.g. browser) dealing with JSString / JSVal is often more efficient than marshalling by way of ByteString (at least for the JS backend). This change should be an easy win, keeping all existing code unaffected (via the type family defaulting rules), but allowing users on different platforms to use a type other than ByteString.

Usage would look like:

-- * when using servant-client on native platforms,
-- default to ByteString (existing code should be unaffected)
instance MimeUnrender JSON String where
  mimeUnrender Proxy = eitherDecode

-- * when using servant-client in the browser,
-- you can specify `JSVal`.
instance MimeUnrender OctetStream Blob where
  type MimeUnrenderType Blob = JSVal
  mimeUnrender Proxy = pure . Blob

[dmjio]

I need to make use of this associated type family in servant-client-core (MimeUnrenderType/ MimeRenderType), to remove the ByteString specific assumptions, then https://github.com/haskell-miso/servant-miso-client can use servant-client-core as well (as opposed to rewriting the HasClient wheel here).

[tchoutri]

@dmjio A changelog entry would be really appreciated for this PR. :)

[dmjio]

Okay so, I am temporarily deciding against this. Ended up making my own MimeUnrender / MimeRender classes to facilitate this instead. My classes are specialized to JSVal (https://github.com/haskell-miso/servant-miso-client). This PR would necessitate a lot more changes to servant-client-core than desired, there are more ByteString specific assumptions than I had previously realized in servant-client-core. Closing for now.