Retrieving the datacon of an arbitrary closure

Retrieving the datacon of an arbitrary closure

This patch extends the RTS linker and the dynamic linker so that it is possible to find out the datacon of a closure in heap at runtime:
- The RTS linker now carries a hashtable 'Address->Symbol' for data constructors
- The Persistent Linker State in the dynamic linker is extended in a similar way.

Finally, these two sources of information are consulted by:

> Linker.recoverDataCon :: a -> TcM Name