Fix wildcard protos

[odersky]

Fixes issue discovered by @cswinter on #1396. Also fixes printing of tuple and function types with wildcards. Review by @smarter.

[smarter]

If I write:

  val foo = (x: (_ <: String)) => 1

Then I get after frontend:

    val foo: _ <: String => Int = {
      def $anonfun(x:  >: Nothing <: String): Int = 1
      closure($anonfun)
    }

Which compiles, but the type of the parameter of the def shouldn't be a wildcard, right?.

[smarter]

I guess my example shouldn't even compile, in scalac we get error: unbound wildcard type

[smarter]

LGTM, opened #1403 to keep track of the issue I noted above