v0.18.0-M1

This release is the product of a long period of parallel development
across different foundation libraries, making a detailed changelog
difficult. This is a living document, so if any important points are
missed here, please send a PR.

The most important change in http4s-0.18 is that the effect type is
parameterized. Where previous versions were specialized on
scalaz.concurrent.Task or fs2.Task, this version supports anything
with a cats.effect.Effect instance. The easiest way to port an
existing service is to replace your Task with cats.effect.IO,
which has a similar API and is already available on your classpath.
If you prefer to bring your own effect, such as monix.eval.Task or
stick to scalaz.concurrent.Task or put a transformer on IO, that's
fine, too!

The parameterization chanages many core signatures throughout http4s:

• Request and Response become Request[F[_]] and
  Response[F[_]]. The F is the effect type of the body (i.e.,
  Stream[F, Byte]), or what the body .runs to.
• HttpService becomes HttpService[F[_]], so that the service
  returns an F[Response[F]]. Instead of constructing with
  HttpService { ... }, we now declare the effect type of the
  service, like HttpService[IO] { ... }. This determines the type
  of request and response handled by the service.
• EntityEncoder[A] and EntityDecoder[A] are now
  EntityEncoder[F[_], A] and EntityDecoder[F[_], A], respectively.
  These act as a codec for Request[F] and Response[F]. In practice,
  this change tends to be transparent in the DSL.
• The server builders now take an F parameter, which needs to match
  the services mounted to them.
• The client now takes an F parameter, which determines the requests
  and responses it handles.

Several dependencies are upgraded:

• cats-1.0.0.MF
• circe-0.9.0-M1
• fs2-0.10.0-M6
• fs2-reactive-streams-0.2.2
• jawn-fs2-0.12.0-M1