Abstract encoding and decoding type classes

[travisbrown]

We currently have DecodeRequest and EncodeResponse type classes, which works well for creating servers. The needs of clients (see #228) are very similar, though, and introducing new DecodeResponse and EncodeRequest type classes would duplicate a lot of code.

Last night I started playing with more generic Encoder and Decoder type classes, and I think it's worth investigating whether that's a viable solution.

[vkostyukov]

@travisbrown Looks very interesting!

I wish we could also get rid of EncodeAnyResponse and DecodeAnyRequest.

[vkostyukov]

I guess we've already moved towards that direction having Encode abstracted over its content type (see #541).

[vkostyukov]

As per (#663) Decode now embeds content-type (a type-level string) making the whole story about implicit decoder/encoders in Finch "more explicit".

@travisbrown do you think this could be closed now?

[travisbrown]

Yep, thanks @vkostyukov!