Explicit types for implicits in tests/untried

Fixes https://github.com/lampepfl/dotty/issues/77 and https://github.com/lampepfl/dotty/issues/82.

This PR applies the [migration tool](https://github.com/samuelgruetter/srewrite) to tests/untried to get the following changes:

For tests/untried/pos:
*    no procedure syntax for secondary constructors
*    explicit types for implicit vals and implicit defs

For tests/untried/neg:
*    no procedure syntax for secondary constructors
*    all files which contain implicits moved to tests/untried/neg-with-implicits

(Note that procedure syntax for functions returning Unit was already removed in a previous PR.)

As you can see, explicit types for implicit vals and implicit defs is still missing for tests/untried/neg, because this is (almost) impossible: In order to add explicit types, the typechecker must have succeeded, but for the tests in neg, it has to fail. That's why I just moved the neg tests involving implicits to a separate folder.