Add tests

Author: odersky

docs/docs/reference/contextual/import-implied.md

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+---
+layout: doc-page
+title: "Implied Imports"
+---
+
+A special form of import is used to import implied instances. Example:
+```scala
+object A {
+  class TC
+  implied tc for TC
+  def f given TC = ???
+}
+object B {
+  import A._
+  import implied A._
+}
+```
+In the code above, the `import A._` clause of object `B` will import all members
+of `A` _except_ the implied instance `tc`. Conversely, the second import `import implied A._` will import _only_ that implied instance.
+
+Generally, a normal import clause brings all definitions except implied instances into scope whereas an `import implied` clause beings only implied instances into scope.
+
+There are two main benefits arising from these rules:
+
+ - It is made clearer where implied instances in scope are coming from. In particular, it is not possible to hide imported implied instances in a long list of regular imports.
+ - It enables importing all implied instances
+   without importing anything else. This is particularly important since implied
+   instances can be anonymous, so the usual recourse of using named imports is not
+   practical.

tests/neg/import-implied.scala

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+class TC
+object A {
+  implied tc for TC
+  def foo given TC = ()
+}
+object B {
+  import A._
+  foo            // error: no implicit argument was found
+  foo given tc   // error: not found: tc
+  foo given A.tc // ok
+}
+object C {
+  import A._
+  import implied A.tc
+  foo            // ok
+  foo given tc   // ok
+}
+object D {
+  import A.foo
+  import implied A._
+  foo            // ok
+  foo given tc   // ok
+}
+object E {
+  import A._
+  import implied A._
+  foo            // ok
+  foo given tc   // ok
+}