Further generification of type signatures in DGM

As discussed on the mailing list (http://groovy.329449.n5.nabble.com/Quick-Check-td2267456.html#a2267980), this patch further generifies type signatures in DGM where it makes sense. Besides, it contains several other improvements and bug fixes to the code, mostly as a consequence of changes in the type signatures or because my IDE went wild. A few usages of generics have been removed because I considered them wrong. Although I double-checked every change, it wouldn't hurt if someone reviewed the patch and provided some feedback.