tags: create entries for class definitions and instances

The tags feature of the itask-tonic compiler should generate tags for class definitions and instances.

If you compile the following program in the iTasksTonic environment, you get a tags file with two entries:

• Start in line 9
• snafu in line 7

It would be nice to additionally have:

• Foobar in line 3
• Foobar in line 6

    module test

    class Foobar a where
      snafu :: a -> Int

    instance Foobar Int where
      snafu x = x

    Start = Start