Overreact allows dynamic content in the map.

This commit provides two major improvements: 1) the concept of reference
is been supported and 2) the interpreter knows how to deal with
heterogeneous values.

Therefore, we don't need to parameterize the program with the type of
content, and we can develop more realistic programs.
All Haskell applications have been adapted to new core.

Minimal corrections in order to have them working with new program
types.
Minor commit: deleted some debugging lines.
Merge pull request #23 from jeslg/existential

Existential