Add Equal constraint

...which also replaces the whole IntVar.__eq__ implementation, so lots
of tests were removed for that.

Added check in Constraint base class to ensure all operands are pyvap
variables (the same-store check would've caught it anyway, but raising
a TypeError is more helpful).

Also fixed up some tests using assertFalse(boolvalue) -- apparently,
those pass even if boolvalue is None (since it does bool()). Oops.
Make Store.search() accept list of variables

Solutions will be unique in regard to these variables. All variables
will still be determined before reporting a solution.

Store.search() now returns the number of found solutions.
Add test for getting arbitrary number of solutions from search