Don't demand explicit types for local implicit vals

[odersky]

A local implicit val can be referred to only from text following the valdef. Therefore, it
does not need an explicit type for type inference reasons. By allowing to elide the type
we make porting from Scala 2.x easier and avoid busywork.

[odersky]

Here's what is currently rejected:

 def f() = {
    implicit val s = "x"
    implicitly[String]
 }

[olafurpg]

To further motivate this change, it appears to be quite common to define local implicit vals. Examples:

• Scala.js https://github.com/scala-js/scala-js/blob/2b34d36f4a698cb4916cdefd4bcb3e0a2eb5d91e/compiler/src/main/scala/org/scalajs/core/compiler/GenJSCode.scala#L601
• Monix:
  https://github.com/monix/monix/blob/07713b5179265022c795a1618d7d0af9b0c34a33/monix-cats/shared/src/test/scala/monix/cats/tests/BaseLawsSuite.scala#L195
• Akka: https://github.com/akka/akka/blob/b8e76586391fdf4c5013bde9198f032906f0b8c1/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowMapAsyncSpec.scala#L50