The empty string is now a recognized configuration filename. It means that
the default configuration with an empty loadpath should be loaded. It also
means it will not be saved.

This removes the need for a WHY3NOCONFIG environment variable and some
tricks based on the WHY3LOADPATH variable. This also removes the need for
a /dev/null file.

warning for possibly useless quantifiers (unless a leading underscore is used)

Remove axioms from int.Power realization.

note: program extraction IS NOT yet implemented

Coq realization for int.Power (mostly to keep Coq proofs that were in power.mlw)

side-effect: function syntax in driver now accepts %v0, %v1, etc.
for type variable instantiations (order is given by variable name)

- fixed "make distrib"
- updated CHANGES
- changed version number to 0.72