Add => as a primitive. Define Not, And and Or using =>. Add MP and Discharge as axioms.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Refactor theorems. Add new theorems.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Reformat code (make formating more consistent)

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add ImpAntisym axiom

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Remove useless is_* functions. We can use equality for that (more readable and similar performance).

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>