Introduce ServiceBuilder

[vkostyukov]

This is an attempt to solve #575 and a last PR in 0.11.

The idea is to use new abstraction ServiceBuilder that maps an HList of endpoints (even coproduct endpoints) to an HList of content-types. When an actual conversion (toService) happens we fold un underlying list of endpoints and apply them one-by-one to a given request until it matches. The HNil case behaves as 404 (which is very nice).

The high-level API is following:

val service = ServiceBuilder()
  .respond[Application.Json](getUsers :+: postUsers :+: deleteUser)
  .respond[Text.Plain](healthcheck)
  .toService

Note that toService still works, but marked as deprecated.

This is probably not the most efficient and clean solution, but I admire its simplicity and how small the diff itself. None of the Endpoint, ToResponse, Encode internals/API were touched - everything is done by new code.

Please, let me know how do you like the API. The only thing I'm worried about right now is whether or not it's good enough for further steps in this direction (content-type negotiation).

[codecov-io]

Current coverage is 74.10%

Merging #583 into master will increase coverage by +0.02% as of 9d92cd2

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[travisbrown]

This is nitpicky, but I think I'd prefer either empty or just a respond[CT <: String] method here that would give you a ServiceBuilder(HNil).respond[CT] (just because Foo() screams "side effects" to me and is kind of a workaround for syntactic limitations in Scala anyway). I'd also be happy to keep it as is, though.

[vkostyukov]

See #794 instead.