Improving the code for declaring ADTs.

[ravimad]

The code in the file ADTManager is somewhat hard to follow with too many nested recursion. It has also been a source of other bugs. In principle, we can separate the algorithm into 3 parts: (a) constructing a dependency graph between ADTs, (b) computing SCC DAG, and (c) traversing them in the bottom up order and declaring the ADTs.