Fix conditionaly compiled code, that contained type errors.

Fix a regression where polymorphic variables would not be chosen in favor of monomorphic ones during the 'single' phase of constraint generation.

This branch fixes several problem with the typechecker and pretty-printer.
It also re-organizes the Types module and extract the Tallying into a separate file.

Improve the debugging of the unify step of the tallying. Fix a bug where substitutions were applied in the wrong order, yielding partially empty types.

Change the ordering use for types during constraint generation with one which is compatible with subtyping, when types are comparable.

 * add a makefile rule 'make .merlin' to generate a merlin project file
 * call ocaml with the -bin-annot if supported
 * add the *.cmt* files to .gitignore and to the makefile clean rule.

Further simplify the type algebra code, using the newly introduced combinators (map/fold/iter) over types.

Move the bool.ml{,i} files in types/ where they belong.

Add more test cases to the the pretty printer/parser test suite.