Deprecate implicit syntax

[vkostyukov]

Addressing #833.

Opinions on other ways to achieve this are more than welcome!

scala> import io.finch._
import io.finch._

scala> val p = get(/) { Ok("foo") }
val p = get(/) { Ok("foo") }
<console>:14: warning: method get in trait DeprecatedEndpointMappers is deprecated (since 0.16): Enable syntax explicitly: import io.finch.syntax._
       val p = get(/) { Ok("foo") }
               ^
p: io.finch.Endpoint[String] = GET /

scala> import io.finch.syntax._
import io.finch.syntax._

scala> val p = get(/) { Ok("foo") }
val p = get(/) { Ok("foo") }
p: io.finch.Endpoint[String] = GET /