Eliminate meaningless to-string conversion

[vkostyukov]

When it comes to Netty -> Finagle -> Finch, the HTTP body lifecycle is ChannelBuffer -> Buf -> String -> CaseClass. If we'd be able to teach jawn to work with Buf instead of String, we'd be able to go directly from Buf to CaseClass thereby eliminating that meaningless to-string conversion, which in fact alllocates O(n^2) bytes.

This would be a huge win for Finch users, but this requires some refactoring around DecodeRequest. The idea of top of my head is to actually keep DecodeRequest a string-based type-class and use it for params, headers, etc (i.e., param("foo").as[Int]) and introduce a new type-class FromRequest = Request => A (as a symmetric version to ToResponse) to be able to grab content: Buf from the request if we need it and not perform the to-string conversion. In this case, body readers should use the FromRequest type-class via the as method.

I'm totally fine to start with Circe (make it decode JSON from Buf) and keep the rest of the JSON lirbaries we support working with strings.

[vkostyukov]

Fixed in #671 (kudos to @rpless).